You can add missing words to persdict.dat in the Firefox profile folder via "Add to Dictionary".

Note that you can have multiple dictionaries enabled at the same time for possible words that are in another English dictionary.

• https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

The fact that so many common words are "missing" is the part that's frustrating. I had to add "missable" today, along with two other words in the Oxford & Merriam Webster dictionaries. Spell check doesn't help much if it doesn't know the majority of prefixed and suffixed words...