It seem's that Firefox intalled by default with Lubuntu 13.10 is not localized (in french for my case).
I performed a clean install ( booting live on Usb key, and then installing, using french language,and choosed not to keep anything on hard disk )
I have "FR" on systray, and on synaptic, I can see that "firefox-locale-fr" is installed ( but "firefox-locale-en" is installed too.
Any clue to get firefox in french ?
 : suggestion from ark : - remove firefox-locale-en has no effect. - language support is already in french. I changed language order to get french canadian just under french and not at end of list... nothing changed..
I thing that something is wrong but just with firefox : other default tools are well localized : pidgin, gnumeric , abiworld...
 : installig "local switcher" plugin : show me only english choice.
problem was solved after installing latest updates : Firefox was automatically switched in french after reboot, and "local switcher" plugin show me more choices : : English US+england+south africa, french