Not having most of those issues here, but to try and help with some:
Some websites aren't working, freeze or videos don't load: this might be due to an extension you're using that blocks ads, or the built-in Tracking Prevention. From the Firefox menu (three-lines icon), click Extensions and Themes to check what extensions you're using, and perhaps try turning one off, closing Firefox, waiting a minute, re-open Firefox and see if any of them are causing the issue. Or for the built-in Tracking feature, go to Menu > Settings > Privacy and Security and make sure "Fix major site issues" is enabled. If you still have issues on particular sites, either turn the "Fix minor site issues" option on, or tap on the 🛡 icon in the website address bar and turn off Tracking Protection for that site.
Around 20 firefox.exe threads open after closing: it's normal for these to take about 10-20 seconds to all close, but if they're permanently there I'd guess either it might be related to the sync feature, if you use that (Menu > Settings > Sync), or maybe for some websites, when they've asked you to display notifications, you've allowed them to, so these processes are to allow websites to still send notifications after you exit Firefox. Go to Menu > Settings > Privacy and Security then scoll-down and find "Notifications" and check if you have sites listed as "Allow" in there you could remove.
If you still have issues, backup your current bookmarks, history and passwords using the new local-backup option (Menu > Settings > Sync > "Turn on backup"). Then once you have a backup created, go to Menu > Settings > General > Profiles > Settings > Manage Profiles, create a new one in the window that pops up, then use Settings > Sync > Restore your data to get your bookmarks, history and passwords back.
