I appreciate the warnings and thank you both, but it is the only procedure that has enabled me to use Thunderbird again. I have used CCleaner from time to time over many years and not been adversely affected in the past. I don't want temp folders or session information clogging my system anyway and the remembered passwords is no big deal compared to being unable to use my preferred mail system.

I know we do have to be careful about this type of thing. I downloaded (I think it was Regcleaner) over a year ago and it disabled my anti-virus which I didn't discover until the BSOD alerted me to all the problems resulting from this. The advice I followed from a reputable online support website to fix the resultant BSOD and virus infections also included suggestions to download CCleaner and Malwarebytes.

This is what I face just about every time I start Thunderbird, but in this event my computer has already up for half an hour, and Thunderbird has already successfully downloaded new mail (though it bogged down, just like you see here). In the instance of this image, it was trying to download a reply to an email I sent.

I use Thunderbird in POP3, letting it first bring messages into the download default folder, then filtering them to the folder I retain messages in. Perhaps this is a factor, I don't know. The messages are stuck in the default folder until Thunderbird can resolve whatever is perplexing it, and they cannot be read until they get to the keeper.

I am NOT using Windows Defender anymore, but rather Webroot SecureAnywhere. This didn't change the behavior of Thunderbird, as far as I can notice. Bogging down on message retrieval is just part of the experience.

Anti-Virus/firewall products perform scans on anything you Run or Open or Download.

So you would need to make sure it is not scanning Thunderbird when it starts up and not scanning any files to do with your Thunderbird Profile files. Stop scanning incoming/downloading mail.

Some people have said, well it switched the anti-virus off and it had no effect, but if you do that then the default anti-virus ('Bitdefender' perhaps) will auto work, so something is still scanning.

Basically, it is likely another program installed on your computer is causing the problem.

Test: 1. Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

2. 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.

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

It's not possible to exclude a folder using Webroot.

I checked online and can see that dispite a massive load of requests to update the product allowing exclusions, they have currently not progressed with this and are likely not to change anything.

So either you have to live with Webroot and it's limitations or uninstall it and then make exclusions in Bitdefender to test.

But have you tried the two tests I mentioned. You have not posted anything back on results of those tests.

No, I haven't tried the two tests yet. I worked all day. I might do that tomorrow.

My reasoning for having Webroot instead of Bitdefender has to do with capabilities and standards. I prefer Webroot.

Just now, as my computer was freshly up and running, I did my usual email check. I downloaded 16 messages (no attachments) in a matter of less than 5 seconds. No hitch. It does this sometimes, even though Webroot (and formerly Windows Defender) are running. And sometimes Thunderbird will bog down in a single message after hours of steady operation (as per the photo I uploaded). I can't explain the normal operation any more than I can the abnormal, but still believe the fault is within Thunderbird.

I took a different tack, and abandoned Thunderbird for eM Client. So far, I have experienced no freezing whatsoever. A few more days like this and I will know the answer.

I had a problem with Thunderbird for weeks after an upgrade from WIN 8.1 to WIN 10. The symptoms were just like those described in many posts. Thunderbirds starts normal but after clicking any button it becomes unresponsive. No error message or anything. It took usually 2 minutes before it 'cleared', was than working fine for a few minutes and bang- it happened again.

After a little research I found out that in some instances, after the WIN upgrade, multiple unnecessary 'MSF' files were in my Thunderbird profile folder. A simple free tool called Thunderfix helped me out of the misery. Now everything is fine and works as it should. I do not have any vast interest in promoting this tool or the developer, just wanted to share the solution I found. This is the link for the tool.

http://www.parhelia-tools.com/products/thunderfix/thunderfix.aspx

re : multiple unnecessary 'MSF' files were in my Thunderbird profile folder.

Additional info: - This info is just for clarification that using a tool to delete an .msf file is not necessary and was purely the choice of the user.

• .msf files are indexing files and do not have any emails stored in them.

Note: there should be one for each folder you have in your Folder Pane.

If you have already gained access to the profile folder and can see those *.msf files, then you can manually delete the .msf files. This must be done with Thunderbird closed. So exit Thunderbird and then delete .msf files from profile folder.

When you restart Thunderbird all of those .msf files will be auto created.

You can also force an index repair on a folder's .msf file via Thunderbird as on occasions it can become 'out of synch'. So, if you think the folder is not displaying all emails it should contain, then forcing a reindex by repairing the folder can resolve some display issues.

• Right click on folder and select 'Properties'.
• click on 'Repair folder' button
• click on 'OK'.

Had this problem so severely I decided to do all email on Linux with Thunderbird and it worked perfectly.

And now, the latest Windows 10 update - Version 1511 Build 10586.71 seems to have solved the problem.

Currently running TB 38.5.1

What changed we will probably never know, but (I write with fingers crossed) TB on Windows 10 has been stable for three days