Rated 3 out of 5 stars

Linux version? Thunderbird is available on Linux, the add-ons are coded in JavaScript and thus the only reason to create plugin limited to group of systems, or just one, is if the add-on needs to access something OS specific.
Which would make me very careful, after asking why on earth it needs to do something OS dependant, then I'd forget the plugin *unless* I had *really* strong urge to have what it claims to offer, after which I would check the source code and/or (in any order) test it on FireFox W32 version on wine tracing it's file and other accesses, but I'm an amateur on that last way.

Rated 2 out of 5 stars

Customizable keys would make this quite good, combos of keys could, and since it already has them, would make it better than most mouseless plugins IF the 1st step is done, customizable keys.

Now what would make this almost as superb as KeySnail (or Vimperator, though ignorant know not KeySnail can be set up with 100%, or at least 97% of Vimperator functionality - with full programmabality, kinda like in eMacs, providing means to program anything you could do with full plugin because the code is javascript, not eLisp ;)) would be creating key-sequences - for those not knowing emacs I give an example: In my firefox (C=Ctrl, M=Alt/Meta) C-x o will jump to another frame (if on framed page), C-x 1 then again opens currently active frame alone as page shown. Most basic are just combos, not sequences though, but ie. C-x is nothing alone, add C-c to end and it quits FF (or emacs, I tried to make them as similar as possible, and especially in editing text fields or vieving mails the basic movement keys, be they Vim's or eMacses heritage and also selection keys, copy/cut/yank, all keys needed on writing area, no need to move hand to mouse or even arrow keys, etc...
KeySnail has loads of other stuff, but I would give a plugin with customizable key combe sequences, if you then implemented a way to attach piece of code, similar you can use with KeyConfig, then it would be gold grade development, and it's NOT that hard.

The rest is not suggestions for mouseless, just mind food of what the most powerful keyboard optimized plugin can support, for example, the features, expandability, etc. - give it a thought, there is no KeySnail for ThunderBird, but you could do a lot to make situation better, even if you just add customizable keys, just regular key/key-combo (1 normal key, rest 1 to 8 modifiers - most I've seen is 3 modifiers, C-M-S-(somekey) - S means Shift naturally, M is for Meta but on PC's often same as Alt. KeyConfig could add this functionality, but I would need to know what function to call to execute to do what mousless does on Ctrl-d and Ctrl-u to bind them to C-v and M-v - of course I should get quite a lot extra functionality to already grand functionality of KeyConfig with Functions for KeyConfig, but those are halfway solutions - if you wan to be the first to implement superior kb-only plugin for TB take some of my ideas, maybe look into FireMacs plugin, lamer version of KeySnail, but if you could port that or create similar taking code/ideas from source design at least you would have seriously possibly the best of the kind plugin, probably also enough for all users, I'm not sure if TB users have necessarily any need for KeySnail for TB, the browsers are different...

Not platinum though, KeySnail has some specialities, but it was made to be extensible, it even has plugin system - but those plugins have advantage of having also KeySnail API and functionality available in addition to being easier to do but still can do anything a FF add-on since for FF it's seen just as part of KeySnail.

Rated 3 out of 5 stars

If you could add the functionality of HaH (FireFox plugin) for HTML mails... that would be awesome!