I can see this happening in my copy of Firefox as well. It looks like this is a known bug in Firefox (https://bugzilla.mozilla.org/show_bug.cgi?id=1228034 ), which happens whenever multi-process support is enabled. Multiproecess support was likely automatically enabled for you when you upgraded, which is why you're just seeing this bug now.

If you want, you can disable multi-process by opening about:config, clicking "I'll be careful", typing "browser.tabs.remote.autostart" in the searchbox, and double-clicking to change the value that appears to "false". However, this will make Firefox noticeably slower, and also a bit less secure, so I wouldn't recommend doing it. May I suggest using the console in the developer toolbox (Tools menu > Web Developer > Web console) instead?

I tried changing the "autostart" value in "about:config". For me, the property that fixed the problem was "browser.tabs.remote.autostart.2". However, I certainly would prefer Firefox to be as fast and secure as it can be. Disabling multi-process support doesn't sound ideal, though it does stop the error count from going haywire.

Also, I do use the console in the developer toolbox. But I like how the toolbar alerts me to errors even if the console is not open. That way, I don't have to browse with the console always open.

Thanks for your quick reply and suggestions!