is there any way to change the FF localization to english, like using the Quick Locale Switcher add-on ?
don't know if it is possible to have a user-dependent FF locale setting but I assume that this is the purpose of the switcher, just try and let us know

The language at the login window corresponds to the current system language, so if you change it back to English it will be English and if you want to have it in Catalan, you must run addlocale to change the system language to Catalan. But please be aware that
- you can never have more than one (1) system language, and as a direct result
- the welcome window and all translatable applications show in the language which was selected in the last run of addlocale
- if a user changes his/her desktop settings to a previously added locale in his/her KDE control center then this will only change the KDE desktop interface language but it will and can not change the system dependent application language.
At present, the shortcut link in the Utilities folder is not correct as it points to an old version of addlocale not having Catalan. In contrast, the link from Menu -> System .. points to the correct version located at /usr/
sbin/addlocale. Please use this version. These small things will be fixed with the new upcoming version.
regards,
-p.