Update manager offers new updates : Firefox 34.0 plus French package - I installed it.
I started Firefox and everything is in English. Tried via terminal to apt-get firefox-locale-fr
, but it says I have the most recent version.
I retry Firefox (still in English) and I presume this is the cause that Firefox does not start, I get error Firefox already running but not responding
.
I backed up my bookmarks and reinstalled Firefox, and the problem was solved.