Why is my spell-checking in Firefox in British English?

[Yora]

This is something that has been bothering me for years. I'm currently using Firefox 76 (on a Fedora 30). In the Preferences, Language is set to "English (United States)" and the only other item in the alternative languages list is "German". The list for Preferred Languages only contains "English (United States)".
In the Add-ons manager, the only Dictionaries are "English United States Dictionary", "German Dictionary", "German Dictionary (Switzerland)", and "German Dictionary (Austria)".

But it still insist that color is wrong and wants to change it to colour.
Favor is wrong and wants to be replaced with favour.
Movie is not a word.

However, it also marks "apologise" (BE) as wrong and wants to turn it into "apologize" (AE).

Is there a way to fix this so I get consistently American English spell checking?

[Rogar Demonblud]

Sounds like you might have a jumbled dictionary. Best to ask at Mozilla and see if it's a known bug with an existing fix.

[factotum]

This may be a silly question, but why do you have various dictionaries in your add-ons? AFAIK Firefox doesn't need those, it just uses its internal dictionaries for each language? Does disabling those dictionary add-ons help?

[Nifft]

Firefox has had an oddly activist spellchecker for some time.

I remember when they decided "staves" wasn't a word -- ignoring that it's a perfectly fine conjugated verb even if you prefer "staffs" as the noun plural.

There are dictionary replacement add-ons which might fix the bug for you.

[Yora]

This may be a silly question, but why do you have various dictionaries in your add-ons? AFAIK Firefox doesn't need those, it just uses its internal dictionaries for each language? Does disabling those dictionary add-ons help?

Because they are automatically pre-installed. They are just in the Addon-Manager because that's where you can manage additional languages.

I just went looking some more and found this 10 year old help request for the very same problem.
And the top solution worked. For some unknown reason, the default language for my system was "English (Malawi)".

Not that I have a problem with Malawi, but what?!

I don't even have a Malawian English dictionary installed. Setting it to United States seems to have fixed the issue.

How does this happen? Why is there no way to see it in the Preferences and language settings? I still blame this entirely on you, Firefox.

[Rogar Demonblud]

It's probably an artifact of what ever mirror you used when you downloaded it ten years ago. Malawi used to be a hosting service base camp due to political things we can't discuss.