Hi, arnon

The language can be change by settings if not working then apply the below one.

You need to find and install a language pack – a special type of add-on – from addons.mozilla.org

You need to download the version you want (see this this page for all systems and languages), uninstall the one you have (no data will be lost) and install the version you just downloaded. http://www.mozilla.com/en-US/firefox/all.html

Thanks

If you want a Hebrew version of Firefox, don't bother with installing a "language pack", just download the proper version to begin with! And on Windows you can install it over whatever you now have.

But when you want to be able to switch between languages, that's when "language packs" come into play.

Use this page - https://www.mozilla.org/en-US/firefox/all/#product-desktop-release - to download Firefox in your preferred language.