I now can't use Thunderbird at all. When I open it, it automatically de-highlights all my emails and then deletes them without any pop up boxes or asking. Should I uninstall and reinstall? If so, will I lose my contacts and emails?

What is "de-highlights" I really do not understand at all.

What I do know is following updates some anti virus products that have firewalls block Thunderbird from accessing the internet and when you use IMAP mail accounts that removes the cached mail from thunderbirds user interface.

One way to test that is;

• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

Sorry, by de-highlight I mean it marks all emails as unread. In other words it goes through every email and every folder and makes all my unread emails no longer in bold.

I ran the program "CCleaner" on my computer and I'm at least able to open up Thunderbird now without getting the pop up message to advance to the next message (and thereby marking all messages as read, even though they haven't been read). All my emails are no longer in my Inbox though. It's possible it might have archived them, but I can't find where if so. I'm not sure it has to do with a firewall blocking anything but I'll try your suggestion. Thunderbird appears to be able to access the internet and download new emails etc. I actually did start it in safe mode after losing all my emails but still got the pop up message and was not able to do anything with my email. After "cleaning" my laptop I am able to at least access my email again, but that doesn't help me with why this has been happening and where all my emails went. I noticed in the community others have had issues with the same pop up message.

ccleaner does not have a good history with Thunderbird, so It concerns me that is apparently fixed this.

I didn't know that. Hmmm. That is strange it helped but is usually not a good fit for Thunderbird. I am using the free version of Cleaner and it definitely removes a lot, but it sure makes my old laptop run better.

CCleaner is well known for deleting files that causes issues in Thunderbird. CCleaner thinks they are temp files, so various important session or even password files can get deleted before other files get updated. It would seem something got corrupted and you ended up with a Thunderbird stuck with corrupted files. Strongly advise if you use CCleaner, make sure Thunderbird folders are exempt. It would not surprise me if the entire issue was caused by CCleaner in the first place.