Firefox isn't the email program that is Google which runs Gmail. Did you check your Gmail settings to insure it's not blocking legit email or mail you get. Also make sure no Addons are blocking Gmail from work as well.

In case the first reply did not already solve the problem, could you describe any error messages you are seeing? For example: is there a problem signing in to your Google account? is there an error when trying to load your Inbox? If it's an error you can copy/paste into a reply here (at https://support.mozilla.org/questions/1391332, not by email), that would be very useful to better understand the problem.

Note that it is possible that Google (Gmail) no longer supports your Firefox 78.15.0 version that is the last for your Mac OS X OS. Does GMail show a link to switch to the basic HTML version ?

Try to clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Settings).

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache