I think you are onto something Azaghal.  However as the Elves are not prone to Illness, Sickness or Disease (or disorders I imagine)  its far more likely that he was touched by the Evil, Hate and Anger of Morgoth.  Who knows, as Morgoth hated Feanor more than any other Elf, perhaps he found a way of afflicting him with some unknown new thing.  He and Sauron managed to create plague like diseases which effected Men in later ages.

There is a heap on Feanor in other threads, take a look ibn the characters section.

My thoughts are similar to what Brego - what for us might be some kind of mental illness, in the Tolkien universe is probably one of many aspects of evil spread by Morgoth on Arda. Feanor's behaviour definitely is something we didn't see before him - I'd say he is showing that particular dark side. It's like seven sins in the Catholic religion - they are all different aspect to the same thing.

I would not say that Morgoth did anything to Miriel specifically while he was imprisoned.

The idea however (see Finwe and Miriel 4, Morgoth's Ring), is that Elvish bodies were made of the stuff of Arda, which contained a Melkor ingredient. Morgoth invested his power into the very 'physical constituents of the Earth' and thus 'the whole of Middle-earth was Morgoth's Ring' 

In this way Morgoth's evil 'entered' into Aman and was in a very general sense a part of Miriel's fate; and I cannot help but note the singular circumstances of Feanor's birth here -- as Nerdanel, for example, had seven sons while Miriel was wearied beyond the desire for life from having Feanor alone.

Like him or not, Feanor was very great, drawing so much from his mother that she was wearied beyond healing, her hroa made of the stuff of Arda (itself tainted by Melkor in the days of his might).

'But in the bearing of her son Miriel was consumed in spirit and body; and after his birth she yearned for release from the labour of living.'

Silmarillion

That is the reason Miriel desired to die. And (for better clarity):

'Also the Eldar say that in the begetting, and still more in the bearing of children, greater share and strength of their being, in mind and in body, goes forth than in the making of mortal children.'

Laws And Customs of the Eldar, Morgoth's Ring

My 'drawing' above was just another way to say this. A great Elf given a notably different circumstance regarding his birth.

 Th

Keep in mind too Feanor's attitude toward Morgoth, and his [Feanor's] way of working:

'... for none of the Eldalie ever hated Melkor more than Feanor son of Finwe, who first named him Morgoth; and snared though he was in the webs of Melkor's malice against the Valar he held no converse with him and took no counsel from him. For Feanor was driven by the fire of his own heart only, working ever swiftly and alone; and...'

And after Feanor crafted the Silmarils, Morgoth: 'From that time forth, inflamed by this desire, he sought ever more eagerly how he should destroy Feanor and end the Friendship of the Valar and the Elves.'

Brego in his first post writes: 'Who knows, as Morgoth hated Feanor more than any other Elf, perhaps he found a way of afflicting him with some unknown new thing. He and Sauron managed to create plague like diseases which effected Men in later ages.' 

But Melkor sowed lies rather than some unknown new thing or disease, if subtly; and conjured visions (in the hearts of the Elves) of mighty realms that they could have ruled at their own will; and by this poisoning (among other lies) he reached Feanor indirectly, for Feanor burned with a desire for wider realms and 'freedom' from the Valar.

Generally speaking this wasn't only something Feanor felt, for even though Galadriel swore no oath, she too (at least) 'yearned to see the wide unguarded lands and to rule there a realm of her own', kindled by the words of Feanor himself.

Of like mind with Galadriel was Fingon, also being moved by Feanor, as well as others of the Noldor, kindled with the desire of new things and strange countries.

What happened to Muriel is not even an uncommon happening today. Because the babe in the womb has to get its nourishment from mummy it occurs at times that the mother seems literally sucked dry in body and consequently in spirit because of the horrible drain so that she may want to just die. This in fact happened to me personally with the birth of my second son, but the first to live. I was having a strange and gradually worsening pregnancy. in the first trimester i had a mild stroke and had to be in a blackened room in the hospital and could not speak and every one whispered around me in case my blood pressure should shoot up.

Things got worse and worse and they had to fly a specialist (heart) in to help me bring my son into the world. I had so many doctors and nurses it was beyond belief. After the birth he seemed unnaturally hungry and he just seemed to take every bit of my strength with each feeding. I was very attached to him, but after about four months i remember clearly having no more strength to fight and just wanting to be released and die. A tough Scottish nurse told me sternly I would surely die if I did not fight. So I was transferred to a Swedish Herbalist who concurred that I was near death and that my son was literally draining me of nutrients and such. I was given treatments over a year and it took two years to recover. My poor little one had to have a bottle instead but slowly I recovered. So, even without any Melkorian shananigans these things can happen. I think that with one at least, perhaps baby John Tolkien, Edith went through a similar horror before things got better.

Elbereth wrote: Ok, then we might think that maybe Feanor has no disease but he was the first Elf that felt hatred.

I won't quibble with the word hatred here, but I'll add that Tolkien does note bitterness among his Elves even before Feanor was born for example [for instance, however taken out of context here for sake of brevity: 'This ill-feeling descended in part from the bitterness of the Debate before the March of the Eldar began, and was no doubt later increased by the machinations of Morgoth' JRRT, Quendi And Eldar, The War of the Jewels]

And Melkor had cozened many of the Noldor before Feanor had reason to 'hate' Morgoth specifically: the Silmarillion notes that many of the Noldor leaned towards Melkor, and (noted later) that the Noldor even began to murmur against the Valar, many becoming filled with pride.

Yes Feanor was among them at this time, but the Silmarillion relates that Morgoth had kindled the hearts of the Noldor to strife, and of their quarrels came at length the end of the High Days of Valinor. Feanor was the first to speak 'openly' against the Valar, and so at first the Valar guessed that he was behind the discontent. 

But ultimately Melkor came to Feanor at Formenos, and Feanor pondered 'if indeed he might yet trust him so far as to aid him in his flight' -- but Morgoth overreached himself and awoke a fire more fierce than he designed, and Feanor then pierced 'the cloaks of his mind' [Melkor's mind] and found there Morgoth's lust for the Silmarils. 'Then hate overcame Feanor's fear...'

And the subsequent slaying of Finwe and theft of the Silmarils... well that didn't help

In any case, one need not imagine Melkor, while captive, doing something to Miriel and Finwe, nor that Morgoth cursed them in Middle-earth before his captivity.

Judging from an earlier post, Brego appears to think so (although I assume it's one or the other at this point, not both] because he finds something problematic with Tolkien's God Eru 'allowing' something, and so Melkor is injected in a way that Tolkien did not employ him.

In my opinion the story needs none of this however. To my mind all one need do is embrace Tolkien's notion of God, Free Will, and Arda Marred, and there is no reason for Morgoth to be cursing Finwe and Miriel before her desire to die. And the author can be quite clear when he wants the reader to imagine a specific curse upon a given person or family -- thinking Hurin, for example.

For the debate of the Valar regarding the case of Finwe and Miriel, see Laws And Customs of the Eldar, Morgoth's Ring. And for an interesting statement from Miriel...

'My life has gone out into Feanor, my son. That gift I have given to him whom I loved. I can give no more. Beyond Arda this may be healed, but not within it.'

JRRT, Finwe And Miriel, version 4, Of The Silmarils And The Darkening Of Valinor, [subtitle Of Finwe And Miriel], Morgoth's Ring

Just now reading Leelee's comment and thinking about music I do believe that J.R.R might have been the first to think of the string theory

Brego, earlier in this thread you suggested that maybe Melkor did something to Miriel during his captivity, then later you suggested he did something to Finwe and or Miriel before his captivity, and more recently you raise the idea of Melkor infecting some of the Unbegotten Noldor in some way.

Can you clarify please: are you changing your mind, or do you think all or some of these things occurred?

And why? I'm still not clear on why you're putting Melkor in a role that goes beyond what the texts relate with respect to his part in the Fall of the Noldor.

As far as I recall, it's not even characterized as anyone's belief (anyone inside the story) that Melkor specifically infected Finwe, Miriel, or any of the Noldorin (Tatyarin) Elves who Awoke, keeping in mind the general matter of Arda Marred, noted in a discussion among the Valar. 

These are all possibilities only, as none of us really know. 

But your possibilities aside, we do know the rather notable part Melkor played in the Rebellion of the Noldor (although not all the Noldor left Aman of course).

Tolkien never specifically denies the possibilities you are raising here, but then again he needn't if he never entertained them in the first place. Arda Marred (a fairly amazing infusion of power on Melkor's part) and Free Will are already part of the equation...

... and JRRT has given (to my mind) ample description of how Melkor undermined the Noldor in Aman. 

Brego, I very much agree. If I remember rightly, even in the Creation Music Melkor was at work, setting everything awry. So, even then he was planning the corruption of Feanor's heart. Just as Satan planned the corruption of Adam and Eve' s hearts.

That is my 2 cents worth. (:

Wen

Galin I don't think, as you stated "Melkor was evil in general"  is correct.  He was made/created by Iluvatar of the same stuff as the other Valar or Ainur.  His evilness grew with pride and envy over millennia.  The Ainur could have existed for millions of years before their music was orchestrated.  Plenty of time for Melkor to change or evolve into an evil being.

I believe this mirrors Feanor, in some ways.  As his pride grew, as I'm sure he was constantly reminded of his greatness by the other Noldor, so did his greed.

I think there was a tipping point for both Melkor and Feaonor.  For Melkor perhaps it was simply jealousy or the feeling of shame after his period of solitude in the Void or maybe his capture and imprisonment in Mandos.  For Feanor it was either the fact that maybe he felt betrayed or at fault regarding the death of his mother or the murder of his Father.

Brego wrote: Galin I don't think, as you stated "Melkor was evil in general" is correct. He was made/created by Iluvatar of the same stuff as the other Valar or Ainur. His evilness grew with pride and envy over millennia. The Ainur could have existed for millions of years before their music was orchestrated. Plenty of time for Melkor to change or evolve into an evil being.

But I didn't write Melkor was 'made' evil Brego. I've been mentioning the concept of Free Will, which allows Melkor to sing in discord.

I said Melkor was evil in general, and I meant very much 'in general', as when Tolkien writes about Morgoth with something like (letter 131) 'the power of Evil still visibly incarnate'. It was just part of a very general, and thus admittedly obvious, description of Melkor.

In short: discord yes, evil yes, Melkor is the great 'Marrer' of Arda even... but of course that doesn't mean he is specifically cursing or infecting Miriel or Finwe, or some Noldo who Awoke, in order to arrange the Fall of the Noldor.