Firefox quite often will remove, replace, add, or layer text as I use it. It has been doing this since I first downloaded it, and I haven't had it for long. (I will say that it isn't the first program I've seen do this. the only other program I've seen do this is Zotero.) I'm not doing anything weird with it, it just happens as I use it. Some of the screenshots I've submitted are of what I'm literally typing right now.

I've tried using troubleshoot mode, but it works just fine when I'm using troubleshoot mode, so I can't appropriately use that mode.