hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Sorry to be so uninformed, but how do I do that? And as far as I know, I don't have any add-ons - how can I check?

the article i've linked to should explain it step-by-step. in the firefox menu ≡ > addons > extensions panel you can see which addons you have present...

So I have feedly nimbus instapaper reader(from Firefox) and a save button (from Pinterest). Do I need them for anything? Should I just disable them? All of them say Legacy.

it looks like the reader one may be causing it. yes if you don't use those extensions (or don't know what purpose they serve in the first place) it's probably best to keep them disabled.

I don't think I use any of them, and as you say, don't really know the purpose. I get to instapaper through a bookmark, I don't use Pinterest outside of the application, and I have no idea what read is. I'm going to disable them all and I bet I won't even notice.

So will the error go away? I do I have to do the update again?

yes, hopefully the error message will go away after you restart firefox once. there's nothing more you'll have to to in regards to the update - you're already on Firefox 55.0, the latest version :))

Just restarted Firefox with no error message. Thanks so much for all the help.