Comment # 1 on bug 1017174 from Andreas Stieger

(In reply to Episteme PROMENEUR from comment #0)
> - in FF i add french and french-fr and i set them to the upper priority

That has nothing to do with the Firefox UI language. This merely influences the
accept-language header that Firefox may send, upon which point web servers may
choose to send you text and other content in that language. 

>  i assume there is something in the environment which indicates system is in
> english.

No. Check if MozillaFirefox-translations-common is installed?