TB slow to download emails, now locks up when I click to open one

Try to start Windows 11 in safe mode with networking enabled.

Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

I followed the steps to start Windows 11 in safe mode with networking enabled. I opened in Safe Mode with Networking enabled, then I opened Thunderbird, but I got messages that "Failed to connect to the server Microsoft 365". It would do nothing beyond that, not even close Thunderbird so I had to restart the computer to close out TB. I retried TB but same problem persists. Ann

Do you have a wired connection to the internet? Networking with safe mode requires a wired connection. WiFi does not work.

No I am not wired to the internet. Uses wifi. But nice to have an explanation. TB is working better today, not perfect, but better than the last couple of days.

OOps! Spoke too soon. TB decided to go slow at opening emails again.

Try to create an exclusion in Defender for the Thunderbird profile folder, so that it's real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

I located and opened the profile TB folder. I found no info on creating an exclusion for a Defender scan. Then I opened Defender and found where I could select a folder for exclusion. I selected folder for exclusion. It brought up a list of folders but not the TB profile folder. I could find nothing for TB. How can I get the TB profile folder to be recognized by Defender in order to exclude it? Thanks

Add the Thunderbird profile folder to the Defender list of exclusions. https://www.youtube.com/watch?v=WCzRJLeSGJg

Until today, I have been delayed in making TB profile folder a Defender Exclusion. Today I was able to exclude it. I verified in Exclusions it is listed, but that does not solve the problem of TB freezing up when I am using it. It works fine for a short time, then it freezes up for anywhere from 5 to 30 minutes, and my hard drive makes a whirring sound until the freezing stops. TB is becoming unusable. Where do I go next?

After excluding the TB profile folder yesterday and still experiencing the freeze up problem, I started using TB this am and it was zipping right along doing an exceptional job. I hoped the problem was gone but after some twenty minutes of using TB, it froze up again, my computer hard drive began whirring rapidly, and after a good thirty minutes of freeze up, it stopped the whirring and TB started working again, zipping along as before. I see in the TB history going back a few years that TB users have experienced this problem in the past with no fix for it that I can find here. Hopefully TB will come out with an update to fix it. My other computer internet websites and outlook server emails work fine while TB is frozen, so they are unaffected by whatever is going on.

See https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems for a comprehensive list of troubleshooting steps.

my computer hard drive began whirring rapidly, and after a good thirty minutes of freeze up, it stopped the whirring and TB started working again

I'm not sure what 'whirring' means exactly, but have you considered this being a hard disk problem?

I have had the same problem, TB will lock up and a message says it is not responding. I got serious about it an uninstalled my anti virus (Avast), malwarebytes and TB. I reinstalled TB. Same problem. I started Windows 11 in safe mode. No change,. I uninstalled the last windows update. No change. My son who is an IT showed me where to disable Windows Defender. I did and TB works like it should. No hangups or slow downs or not responding. I don't know what happened but Windows Defender is the problem.

I hope someone can figure out how to get this fixed. Otherwise, the answer is to shut down the antivirus in Windows Defender every time the computer is restarted since it auto turns itself on at start up.

I ... uninstalled ... TB. I reinstalled TB.

Re-installing Thunderbird usually is a waste of time, since the problem is more likely to be in the profile, and not the application itself.

I uninstalled the last windows update.

It's nothing I'd recommend.

My son ... showed me where to disable Windows Defender.

That's a bad idea, as it would eliminate your entire anti-virus protection. A more secure solution has been posted earlier in this thread.

Thanks Mike 866 and christ1. I read all of the troubleshooting steps and have been trying to identify any specific TB action that might cause the freeze up but have not seen a pattern unless I use TB too quickly, i.e. deleting emails, moving emails to folders, changing Inbox and then coming back to my Inbox. It inconsistently works well and then freezes up. And the whirring begins which I don't think is my hard drive since I bought the Lenovo computer less than 2 years ago pre loaded with Windows 11 and it worked great for the almost 2 years until a few months ago. Earlier this week it literally locked up and whirred for hours at a time. Restarting and/or shutting the computer down did not clear the freeze up. I have also tried starting in Safe Mode and it worked for about an hour making me think that fixed it but then it froze up again in Safe Mode, whirring away. If the problem is Windows Defender for TB I agree with Mike 866 that hopefully a fix is coming. This week it got so bad I was forced to go to the Outlook server and use it, it works fast and fine but I much prefer the TB format for user friendly displays. I also have an IT relative who suggested a fix he remembered from some years ago. I have not yet had time to try it. I will copy his suggestion below: Ann

Ann, I found the issue that I had run into with Thunderbird locking up.  The resolution instructions are listed below.  It tell us to delete the .mfs file in the “mail” and “ImalMail” folders, but I would also look through any other mailbox folder that you see such as outbox, archive, etc. and delete the .mfs files in those too.  However, I would only delete in a single folder at a time, repeating the entire process for each ,folder/.mfs file that you find.  In other words, run through the instructions shown below deleting one .mfs file, reboot your computer, restart Thunderbird, and see if the lockup is resolved.  If Thunderbird starts without delay, I’d stop deleting .mfs files at that point.

Let me know if you run into any problems.

Sometimes the index files that Thunderbird uses to display your emails get corrupted, leading to lock-ups. [1]

   Completely close the Thunderbird application.
   Press Windows Key + R and type %APPDATA% to open your data folder.
   Navigate to Roaming > Thunderbird > Profiles and click on your profile folder.
   Find the Mail or ImapMail folders and locate the mail account causing the issue.
   Delete the .msf files associated with your large folders (e.g., Inbox.msf). Thunderbird will safely regenerate fresh, uncorrupted index files the next time it launches.

For some reason a portion of his possible fix did not copy over into the previous email I sent. Here it is in the entirety: Ann

Sometimes the index files that Thunderbird uses to display your emails get corrupted, leading to lock-ups. [1]

   Completely close the Thunderbird application.
   Press Windows Key + R and type %APPDATA% to open your data folder.
   Navigate to Roaming > Thunderbird > Profiles and click on your profile folder.
   Find the Mail or ImapMail folders and locate the mail account causing the issue.
   Delete the .msf files associated with your large folders (e.g., Inbox.msf). Thunderbird will safely regenerate fresh, uncorrupted index files the next time it launches.

To rebuild the index file of the troubled folder: Right-click the folder in the folder pane - Properties - Repair Folder

This does the same as deleting the .msf file(s). I wouldn't expect this being the solution for your problem though.