Discourse and Firefox

Is anyone experiencing a bug in Firefox on Discourse when editing posts where the up and down arrows often cause a 1-2 second delay when leaving a paragraph? It happens on all Discourse sites that I’ve tried, including this one.

I described it in more detail over here: https://meta.discourse.org/t/arrow-keys-in-firefox/204172

I’m not sure if the bug is in Discourse or Firefox, but I’m worried that all the users on Firefox who use my forums aren’t having an ideal experience. It seems like there are a lot of Firefox users here who might be able to help narrow it down.

Edit: it looks like a Firefox bug, because I can reproduce it on other sites like Pastebin, and even with plain HTML though it only occurs very rarely with the plain HTML. I posted more info in the page I linked to and will see if I can file a Firefox bug report.