Hi, is that showing up on a trusted site? It could be a rogue add-on. Try Firefox's Troubleshoot Mode. In that mode, Firefox temporarily deactivates extensions, hardware acceleration, any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is running:

You can restart Firefox in Troubleshoot Mode using either:

• "3-bar" menu button > Help > Troubleshoot Mode...
• (menu bar) Help menu > Troubleshoot Mode...

and OK the restart. A small dialog should appear. Click the Open button.

If Firefox is not running or menu access is blocked:

Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.) A small dialog should appear. Click the Open button.

Note: Don't use the Refresh without first reviewing this article to understand what will be deleted: Refresh Firefox - reset add-ons and settings.

Any improvement?

Ref. Diagnose Firefox issues using Troubleshoot Mode

If Firefox runs normally in Troubleshoot Mode, a likely culprit is an extension. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• click the Extensions button (puzzle piece) on the toolbar > Manage Extensions
• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. On the right side, find the "Manage Your Extensions" heading.

If there is at least one extension before the next heading -- "Recommended Extensions" -- please continue:

Then cast a critical eye over the list below that heading. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove). For your privacy and security, don't let mystery programs linger here.

Any improvement when you eventually Exit out of Firefox and start it up again normally?

please help me remove/ stop these non-stop 'pop-up 'notifications , saying, they are from Firefox, with a Firefox logo,

That popup is not from the Firefox web browser nor from Mozilla. Anybody can create such a popup image with a Firefox logo or name somewhere in popup in their attempt to trick people into going to insert website or downloading and installing a actual virus or trojan etc. Though your screenshot has no mention of Firefox anywhere in the so called popup.

See also https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Now if these were small popups on the bottom right corner of Firefox windows then it would have been due to web push notifications from sites you have allowed to send you notifications. Instead of useful information the sites may instead send you spam, junk or fake virus/trojan alerts. https://support.mozilla.org/en-US/kb/push-notifications-firefox