To display Web pages in italian if they are accessible in this language
In Preferences page (Edit / Preferences), click on Language button Choose
In the popup:
- select language to add (Italian),
- click Add,
- set prefered Language order,
- click Ok
Leave Firefox
To display menus, … in italian
Launch System settings
In Personalization / Regional settings / Language
- Add language italiano,
- set prefered Language order,
- Apply
Start a new session
Start Firefox, goto Preferences page and select Italian Language
It should work!