I have tried everything I can think of for changing the accept-language of Firefox, but it seems to be stuck on en-GB. I've changed the locale of my system to en-US but Firefox still sends out en-GB. I tried your addon to fix it and it still wont change. I will be willing to give higher marks if you can show that your addon is fine and that it's something else.
Yeah your addon is just fine I think. I'm blaming this one on MOZILLA FF. the addon wasn't changing anything in the accept-language field until I changed "en-US,en" to just "en-US"
The Accept-Language header accepts a list of possible languages... "en-US, en" is probably superfluous, but something like "en-US, en;q=0.9, fr;q=0.8" would mean "I want American English most, then any other English, but would accept French if English isn't available."
There's more info on MDN at https://developer.mozilla.org/docs/Web/HTTP/Content_negotiation#The_Accept-Language_header