''GMail settings state this:''
Desktop Notifications: (allows Gmail to display popup notifications on your desktop when new chat and email messages arrive) Learn more Note: This browser does not support desktop notifications. To enable notifications, upgrade your browser to Google Chrome.
If you are talking about popup windows, try going to Tools, Options and click on Content at the top, and then add Gmail URL as an exception to blocking popups.
Maybe that will help.
It is also possible that since Gmail is part of Google, the Google browser (Chrome) is the only internet browser in which the notifications will work.
Ask a question about this at Gmail. There is probably a help form there, somewhere.
This is the Add-on you need: https://addons.mozilla.org/en-US/firefox/addon/web
You need to update the following. The Plugin version(s) shown below was/were submitted with your question and is/are out of date. You should update to avoid known security issues with the version(s) you have installed. Click on "More system info..." to the right of your question to see what was included with your question.
Still don't get how to receive Google Mail alerts.
What URL and where exactly do I have to add to HTML Notifications to get it running?