Spell check - not even close
Using Thunderbird 37.7.0
Perhaps instead of adding bells and whistles, you might work on improving the spell check. Guess what I am trying to spell - 'recepticals'. Think you got it? Here is what Thunderbird thinks I am trying to spell 'sceptically'. I am trying to spell a word that starts with an 'r' not an 's'. I might be a bad speller but Thunderbird isn't even close. You probably got the word right; it is 'receptacle'. Notice how close I am to getting the word spelled correctly while Thunderbird seems to be flying up a colon somewhere. If I where in the UK then I would really be 'sceptical' of the intelligence behind the spell check algorithm. But I am in the US, so naturally I am more skeptical than sceptical.
Now lets add insult to injury: While Thunderbird suggests 'sceptically' for 'recepticals'. If I type in 'sceptical', it thinks that is spelled wrong! Which it wouldn't be if I were in the UK.
This issues has occurred with different words.
Wšykne wótegrona (1)
I suggest you file a bug. https://bugzilla.mozilla.org/ The search algorithm for misspelt words is way more than this support forum is equipped to work with.
Note that the bug may have to be filed for core as the spell checker is common code with firefox I think.