"PARSERERROR" When Loading Articles & Feeds

For the last couple weeks, I’ve been occasionally getting a “PARSERERROR” when trying to load feeds. Instead of loading an article (or the list of articles), nothing appears except the error message. Screenshot below. I’ve been getting these parser errors in a variety of feeds - it doesn’t seem to be site-specific.

I’m on Firefox 59.0.3 and use HTTPS Everywhere, uBlock, and Privacy Badger, so they could be causing problems, although I’ve always used them and this is new.

Can I provide anything else to help troubleshoot?


Update: with all the of the above-mentioned ad-ons turned off, this error is continuing to appear.

I bet it’s related to a browser extension. I have never seen a parser error there.