In Firefox, if you go to Gmail -> Settings -> General (https://mail.google.com/mail/u/0/#settings/general), does it say "Notifications have been disabled in this browser."?

Check Firefox Settings -> Privacy and Security -> Permissions -> Notifications. Is mail.google.com in the list?

If it is, make sure its status is set to Allow.

If not, make sure "Block new requests asking to allow notifications" is unchecked. Then reload Gmail and in the notification settings you will have the option to "Click to enable desktop notifications for Gmail."

@Mark Thanks for the detailed response! I've already tried all of this, and unfortunately my GMAIL insists on not sending notifications in FIREFOX but only in CHROME and EDGE. Does it work for you?

Yes, it works for me. I heard the notification sound when I received a few emails in Gmail.

What does it say in "Desktop Notifications" of Gmail's settings? I redid the process, first you click on "Click here to enable desktop notifications for Gmail" (Screenshot 1), then Firefox should give you the option to allow notifications. (2) You need to make sure you aren't blocking new notifications requests in Firefox settings.

@Mark Thanks for the reply! The screenshots are very unclear, could you please upload clearer ones? Where is screenshot 1 from? Where is screenshot 2 from?

I understand, now it's fixed! Thank you very much!