The issue where the window needs reloaded after some time does sound like software - specifically like a slow memory leak.
Plenty of us are using Cinnamon without that issue, but of course it could be you have some strange unlucky combination of hardware, patch version, etc.
I'm not sure b if the video thing is related. Pop-out videos come in many flavors, and most range from mildly anti-user to outright evil. So the video misbehaving could be down to the specific combination of site, site video digital rights management, how much spying they try to do while the video plays, how much of that spying Librewolf is set to block, and what day of the week it is (how fast your Internet connection is, that day, really).
Do you have any window manager extensions installed? (I know gnome has a whole extension ecosystem, but I'm not sure Cinnamon even supports Desktop extensions. If you don't remember adding anything, ignore this, it's a really long shot.)
You could try adding a second desktop environment (MATE is a popular enough 2nd choice) and see if the issues happen there, as well. You can switch desktop environments in a drop-down on the login page.