Sorry, if you mean the user interface language, you most likely need to install an English version, which should replace the existing version.

Not sure if you install from your distribution's repository or from Mozilla. If you need an installer from Mozilla:

https://www.mozilla.org/en-US/firefox/all/

The System Details list shows you use 'as' (Assamese).

• https://support.mozilla.org/1/firefox/57.0.4/Linux/as/

Ubuntu can come with a lot of locales. If you want to keep using the Ubuntu version then check general.useragent.locale on the about:config page.

Note that Firefox 58 will use the hidden intl.locale.requested String pref.