Changing HTTP_ACCEPT_LANGUAGE
Also you can go to about:config in the address bar and modify the preference
intl.accept_languages
The format is comma-separated list of language tags. e.g.:
en-us, en-gb, en, es
with the languages listed in decreasing order of preference.
Tools -> Options -> Content -> Languages -> Select a language to add
Yes, you can change the list of preferred languages in Firefox's Settings.
Mozilla Forums: How to change the default language in Firefox