Hello qorval,

My apologies if this is a rethorical question, but after you changed the language, did you then scroll all the way down and click the "Save Changes" button  ?

See :

Change your Gmail language settings

Maybe there's a possible solution contained in your answer. In my Firefox I only see Firefox settings, in which my chosen language is English, and which did not require to be saved. There's no save button anyway

I can't see GMail settings at all, so I can't change languages (in the top right corner of anywhere else). And if I don't see the settings at all I can't change the GMail language.

Any idea where I can find GMail settings?

qorval said

Any idea where I can find GMail settings?

Sure : Open your Gmail account and click on the cogwheel (on the right side).

You'll get a dropdown and there you click on "Settings" ......

See : Change you Gmail settings

Does that help  ?

In case you don't know what a cogwheel (aka : gear) looks like ......

Could be a cookie problem. Did you alreaady cleared the Gmail (google) cookies?

Clear the Cache and remove the Cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" from websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Manage Data

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems