how can we customise Firefox for regional language?

You can see all the languages that Firefox is available in at http://www.mozilla.org/en-US/firefox/all.html

Hi kunjeshgajjar8, maybe the next add-on is interesting for you too.

Padma add-on

thank you

You probably need a rooted device to be able to do that.

See e.g. (Google: Hindi or Gujarati android):

• http://www.megaleecher.net/Devanagari_Hindi_Font_On_Android