As Mozilla is a secured site, such should not happen. But let's try;

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

I have cleared the cookies and cache, but this has not resolved the issue. In fact, the notification from Bitdefender seems to come up no matter which browser I use to access support.mozilla.org.

I doubt that it is malware, as the notification has been coming up almost since I first installed the OS on the machine, and I'm pretty careful about what websites I access. Additionally, I assume malware would not wait for me to visit one particular website (which I might never visit) in order to operate - I would think it would try to contact its target website as soon as it possibly could.

The item being flagged by BitDefender is actually the same one that is mentioned in comment 9 of this Bugzilla page:

https://bugzilla.mozilla.org/show_bug.cgi?id=1300821

Have you contacted BitDefender support? Also, make sure its programs and files are up to date.