How to get firefox language setting? (REALLY)

Both navigator.language and the HTTP User-Agent header use the value of the preference "general.useragent.locale", which is hard-coded in intl.properties to the locale of the Firefox build you downloaded: http://mxr.mozilla.org/mozilla-central/source/toolkit/locales/en-US/chrome/global/intl.properties#8

Or for your en-GB build: http://hg.mozilla.org/l10n-central/en-GB/file/88dd673c01f1/toolkit/chrome/global/intl.properties#l8

If you'd like to change it for your build, just load about:config, find general.useragent.locale, double-click it, and change the value.

I would assume that navigator.language returns it's interface language (I assume is GB English), not one that it wants documents in. User-agent string too report which version of firefox was downloaded.

Accept-language is what destination server should check.

I consider this behavior of his quite correct (assuming that it's interface (menus and such) are really in GB english).

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.