It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All Of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

I tried that initially. I managed to solve the issue by uninstalling and reinstalling Firefox. The same add-ons were present in the new installation.

There may have been a site trying to infect my computer with malware via media playback. Youtube would again be working, but if I tried to use the site in question at the same time to load a webmd, youtube would stop it's playback and not function until I restarted Firefox.

Does the problem remain if you disable the addons?

1911rdk said

There may have been a site trying to infect my computer with malware

Let's make sure your system is okay.

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.