Can you associate this error with a particular website, or does it seem to occur on many different websites?

If it's one particular website, a volunteer could take a look at it if you post the address -- assuming one doesn't need an account to use the page.

If it occurs on many different sites, it could be that one of your add-ons is no longer compatible with Firefox after an update. As a quick test to diagnose that, could you try browsing for a while in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Do you still get the messages or do they stop in Safe Mode?

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?