Considering you are using macOS and not Windows there is much less chance of you having malware.

Are you getting small popups on bottom corner claiming you have viruses or malware or something that McAfee found?. If so then you may be just getting Web Push Notifications from a website(s) you have allowed to send notifications.

https://support.mozilla.org/en-US/kb/push-notifications-firefox https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-web-push-permissions-for-a-specific-site

Maybe by small you do have some malware see https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Btw if you do indeed have macOS 10.13 then the most current Firefox you can use is Fx 115.13.0esr with 115.15.0esr likely to be the last update in September. https://www.mozilla.org/firefox/all/#product-desktop-esr

Fx 116.0 and later including Fx 128.0 requires macOS 10.15 (ten.fifteen) at minimum to run.

I need more help! I’m afraid to open Firefox on my desktop until you help me erase my bookmarks. That’s where, I think, the malware started or is. When I clicked on an old bookmark my screen froze & a voice over repeated that my computer had been breached & wouldn’t stop until I called Apple. They helped me turn the computer off. But before I can turn it back on, I need YOU to help me delete that bookmark (or all bookmarks) from my desktop. Please help….I cannot use my computer until you help me resolve this problem.

qatarkat, did you call Apple via a number provided by the alert itself? If so, then unfortunately there is a chance that the alert was a scam and the people you called were scammers and not Apple? I am not familiar with Apple devices enough to know how likely that would be?

The first thing you should do is run a full system malware scan.

You can find further information on detecting and removing malware here: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware