HI Could be various reasons. Do you find there is lag if you use webmail? Internet pressure due to all the COVID effects like working at home, has caused massive pressure on internet networks. Seville, for example, was recorded by KASPR at 18 times normal in the first half of April. So, it might be local internet or your provider. That is especially true if you re using IMAP - where many actions in Thunderbird involve accessing the mail server - to view folders, view emails, delete or move emails etc. Can you open settings or create filters without lag? If everything is slow, it is probably not the virus checker - that would affect incoming and outgoing messages. Please test which Thunderbird actions have lag and which do not - and report back so we can guess with more details! Thank you.

Open Windows Defender dashboard, click Virus & threat protection, Virus & threat protection settings, Manage settings, Exclusions, Add or remove exclusions, Add an exclusion, Folder, browse to the TB profile folder, typically at C:\Users\username\AppData\Roaming\Thunderbird\Profiles\8characters.default

HI Could be various reasons. Do you find there is lag if you use webmail? Internet pressure due to all the COVID effects like working at home, has caused massive pressure on internet networks. (...)

First, thank you Agnes, for the reply.

It's not internet lag, its the interface that lags/hangs/freezes after a user action, you click something and its like time has stopped, and from 1 second to minutes later a line gets highlighted, a button gets pushed, or an email gets opened. Its starts at random times, goes away only with a system reboot. Very stressful.

Open Windows Defender dashboard, click Virus & threat protection, (...)

Thank you Sfhowes, for the reply. I just applied this one on my system now, and I intend to come back in a month to confirm that no more problems occurred in Thunderbird. If problems arise again, I'll be here too to report it.

I see you're a Top Contributor, so you must copy and paste this one a lot, so I took the liberty to clean it up for you and made some updates to the names of things, so less experienced users can follow it with zero stress:

1) Open "Windows Security" dashboard;

2) On left side menu click "Virus & threat protection";

3) Middle of the screen, under "Virus & threat protection settings", click Manage settings;

4) Scroll down to Exclusions, under it click Add or remove exclusions;

5) "Add an exclusion", Folder, browse to the Thunderbird profile folder, typically at C:\Users\USER_NAME\AppData\Roaming\Thunderbird\Profiles\RANDOM_NAME.default\

Thank you mate.

Oh, by the way: Isn't it incredibly dangerous to disable the protection of that folder?

Oh, by the way: Isn't it incredibly dangerous to disable the protection of that folder?

The real-time background scan provides sufficient protection.

See these articles for more information:

https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues

http://kb.mozillazine.org/Antivirus_software

Well, 2 weeks have passed and the method of disabling Virus & threat protection only for Thunderbird, seems to work. No more slowdowns and interface lag. Thanks!

I literally *just* applied this solution, and within 30 seconds, Thunderbird's UI froze again. And within a few seconds of coming back, it froze again. Then again... Do I need to reboot or restart Thunderbird before the solution takes effect?

In my case, these freezes started with Thunderbird 78.0, and have continued with the few updates that have been released since. Previous versions of Thunderbird didn't have this problem with Windows 10's built-in security suite.

If this continues tomorrow (if I can ever successfully shut Thunderbird down and go to bed, as I've been trying to do for 20 minutes now!), I'll have to revert to version 68 and prevent it from updating again. :(

Does it happen with Windows started in safe mode?

I literally *just* applied this solution, and within 30 seconds, Thunderbird's UI froze again. And within a few seconds of coming back, it froze again.

Are you using WIndows Defender? Does your problem stop if Windows is started in safe mode?

- win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Are you using WIndows Defender? Does your problem stop if Windows is started in safe mode? - win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

I am using Windows Defender, and I set up an exception for the Thunderbird profile folder. I've tried Thunderbird in safe mode, but I have not yet tried Windows 10 in safe mode. I'll try that shortly. Unfortunately (or fortunately?), the freezing is random, and I can sometimes go an hour or more without it happening (and other times, it happens very frequently), so I need to set aside time to hang out in Safe Mode for a while to gain confidence that it's not still happening.

Does it happen with Windows started in safe mode?

Yes. Same problem with Thunderbird safe mode and Windows safe mode. I also replaced Windows Defender with Avast, and the problem continued.

I ended up having to reinstall Thunderbird 68, which resolved the freezing problem... and unfortunately deleted my address books, with around 20 years of gathered email addresses, because TB 78 changes the address book format.

I still need to reinstall TB78 so I can export my address books to CSV files, hope that it saves all of the information, uninstall TB78, and reinstall TB68, and hope that importing the CSV address books retains all of the information.

This new version of Thunderbird is tremendously disappointing. :(

Thunderbird 78.2.0 appears to have resolved the problem. I've been running it for a few days without incident, though it's not clear from the release notes which bug was the culprit.

Thunderbird 78.2.0 appears to have resolved the problem. I've been running it for a few days without incident, though it's not clear from the release notes which bug was the culprit.

That would be the address book changes.