If you want to change the font in all of Thunderbird’s UI, you must perform this magic ritual:
- Create the file
chrome/userChrome.cssin wherever they’ve hidden your profile folder (for Ubuntu 14.04, it’s~/.thunderbird) - Then put this incantation inside:
/* Global UI font */
/* may need !important on each entry */
* { font-size: 14pt ;
font-family: Arial Narrow ;
}
As nearly as I can tell, you don’t need the !important tag on the top-level entry, but I don’t profess to grok Mozilla-flavored CSS.
Useful properties:
font-weight: normal | bold | lightfont-style: normal | italic | oblique
Maybe you can do the whole font thing in one shot, but I haven’t tried.
The changes take effect the next time you fire up Thunderbird: dinking with this stuff gets tedious.
This is way too intricate for mere mortals…
The Smell of Molten Projects in the Morning 
Do you have a way I can prune my unused emails from the suggestions Thunderbird offers me when I’m entering a To: address? It insists on including obsolete ones. I don’t even know if they still exist in some address book.
As nearly as I can tell, Thunderbird rummages through your real address book and the list of addresses harvested from incoming & outgoing emails: you must hunt each suggestion down and kill it individually.
But that’s not always the case. A year ago, I tried to get folks to stop using an email address so I could shut it off. Even after they changed the address book entry, their MRU list kept offering that ancient address; I may be stuck with that address forever. [sigh]