I know this question has been answered before, but the answer given is not the complete story:
I went into Firefox's Options->Content and removed all languages except German/Germany, and navigator.language hasn't changed - it's still en-GB (I'm in the UK).
I'm told if I get the German Firefox INSTALL it will work, but I shouldn't need to do that, right?
The useragent string still contains en-GB, too; but the accept-language on HTTP headers IS set correctly. So this seems to be a bug in Firefox, I spent a bit of time wading through their bugzilla, but I can't see this exact bug logged, though to me it seems a pretty huge oversight?
I would recommend to use a Firefox Add-on for this task. Simple Locale Switcher, Locale Switcher or Quick Locale Switcher etc. (* Caution some of this Add-ons may contain ad ware or may not work if you've just updated firefox).
*Another solution is to download a different old firefox version using the language/locale. (Caution) You need and install it in a different folder so you won't overwrite your existing version. Download here I'm using firefox 28 in Spanish. I disabled updates and I rejected using it as my default browser. Only one firefox version should be running at the same time if you do this.