Mozilla Thunderbird Freezes

We've been using Mozilla Thunderbird for many years without issue until recently when Thunderbird started freezing. There are numerous causes for Thunderbird freezing. However, we do believe that Thunderbird freezes due to 100% CPU usage. Let's take a look at some possible fixes we discovered while troubleshooting this issue.

We should note that initially, we closed Thunderbird when this happened, and that solved our problem for a day or two. If you don't have time to troubleshoot, right click on the Thunderbird in the system tray and select Close Window and restart Thunderbird.

1: Update Thunderbird

We were surprised to discover we had missed a recent update. If you're lucky, an update fixes the problem. Go to Help, About and if an update is available you will be notified here.

2: Verify Your mail.db.idle_limit

Thunderbird goes inactive after 5 minutes of inactivity so if you've noticed that your freezing issue happens after Thunderbird has been idle, then here's a little-known fix. This also applies if you have very a lot of messages because Thunderbird is loading them after you've been inactive.

Go to Tools, Options, Advanced. Look for Config Editor at the bottom. Click on I accept the risk! when you receive the warning that you might void your nonexistant warranty. This change we're making can easily be reversed later.

Type in mail.db.idle_limit in the Search at the top. Note the value. If your value is set to 300000, then you need to add a zero so that it's 3000000. Another way of looking at it is if it has 5 zeros, you want 6. Double-click on mail.db.idle_limit to change the number and close when finished. No restart is needed.

3: Check Your Antivirus

Many users report that bundled antivirus programs including Norton's and McAfee are memory hogs and stripping Thunderbird of needed memory. It's worth a try to disable your antivirus temporarily to verify that's the problem.

If that's the problem, then consider another antivirus or, better yet, uninstall your antivirus and let Windows Defender Security Center take over, which it will so automatically once an antivirus is removed. Windows Defender does not scan email attachments. If you've heard that Windows Defender wasn't any good, that simply is no longer true. We discussed this in detail - What's the Best Antivirus and Is Windows Defender Good Enough.

4: Check Other Settings

We think that the first three steps should have solved most problems. But various settings can be changed if your issue isn't resolved at this point.

Add-ons. If you use any add-ons, make sure they're up to date and still being developed. Older, no longer developed add-ons may not work properly with the latest Thunderbird. Click on Help, Restart With Add-ons Disabled. Click Restart when prompted. If the freezing stops, you'll need to disable add-ons one at a time until you find the culprit.

Clean junk files. If you haven't cleaned junk files in some time, some users report success by using CCleaner. You can download CCleaner here, and a video tutorial is embedded on that page if you need help.

Last but not least, there are quite a few settings under Tools, Options worth looking at. Browse through each tab but in particular the General, Chat, Privacy, Security, and Advanced tabs. You can change anything you like but here are a couple of settings known to cause problems.

General - you can disable sounds, notifications, and even the splash screen to speed up Thunderbird.
Chat - If you don't use Chat, uncheck everything.
Privacy - Much like the General tab, these settings are personal preference but if you don't use a particular feature, uncheck them. If you have a lot of cookies, you could remove them here. Thunderbird often asks if you want to load remote content in messages. If you use this feature frequently, you might want to clear this list under Exceptions.
Security - There's not much here I would play with. However, there is a setting under the Antivirus tab that allows your antivirus to quarantine emails. If you suspect your antivirus is the problem, make sure this is unchecked until you're sure.
Advanced - The Advanced tab has five more tabs worth looking at. The only setting here that some report helps is disabling hardware acceleration under General if it's enabled.

If you know of anything else that's worked, let us know and we'll update this article.