Still no change, but I removed the site extension on the FireFox and kept it on Chrome. It's working fine now. I just can't make certain edits to items with the site extension, it seems.
Yeah, there was a problem with how I implemented the textbox in the item editor (when enhanced plain text is turned on). I’ve already made a fix for both; you can get the updated FF version here, and the Chrome store update should be forthcoming.
Resolved. Thank you.