And stupidly, you can't delete words from dictionary. What a devil.

Hello! If you either (1) click the three-bar menu on the top right --> options --> advanced or paste "about:preferences#advanced" into your browser for Firefox.

You can disable "check for spelling as you type"

You can always add those words to the spell checker.

See also:

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

• http://kb.mozillazine.org/Dictionaries
• http://kb.mozillazine.org/Spell_checking

FredMcD said

You can always add those words to the spell checker.

This doesn't work because dictionary is only finite and there are building number of words I use. However, disabling it "as I type" worked.