Firefox has come to believe that my default spell check language should be Spanish. My global preferences have English selected:
Preferences->Content->Languages->English [en]
and on a page-by-page basis I'm able to reset the spell checker language via:
Right click->Languages->English (United States)
However, for newly opened pages or new sessions the default spell check language returns to Spanish. I found a workaround here: https://support.mozilla.org/en-US/questions/975459#answer-494574
Which suggested that installing a new dictionary would change the default. However, this issue has bugged me for so long that I would like to know if there is a more "correct" way to change the default spell check language.
If you only use system wide spell checking dictionaries (for example, the hunspell package provided by your Linux distribution) Firefox seems not be able to remember the default configuration (is this a bug?). You can fix this issue by having at least one dictionary installed as a Firefox plugin.
Afaik, there is no UI for this. Instead you have to use about:config and set dictionary.spellchecker.
Also, Content > Languages has no bearing on spell checking. This effects what content you're given on multilingual websites.
来源：https://stackoverflow.com/questions/26936792/change-firefox-spell-check-default-language