Change your Mozilla Firefox's Language just simply copy these steps (With Photos)
Steps:
1. Open your Mozilla Firefox browser.
2. Click on "Firefox" in the Firefox Menu.
3. Click on "Options".
4. In "Options" window select "Content" tab.
5. Click on "Choose" tab in the section of "Languages"
6. Now, Select your Language and Click "Add".
7. Click on "OK"
8. Again Click on "OK" and Restart your Browser.
Its Done.... The language you selected will now be your default browser language in Mozilla Firefox.