This has been a bug for a long time but devs have never reproduced and don’t know what causes it.

https://bugzilla.mozilla.org/show_bug.cgi?id=2034677

Users report that it fixes after some kind of restart. You can try, in order:

• Restarting the browser.
• Restarting the browser in “troubleshoot mode” then restarting it again into normal mode.
• Restarting the whole computer.

I had a similar issue yesterday with PDF’s being completely blank in Firefox. Is this related?

Elements blocked. Mission accomplished.