My windows updated yesterday. So this is definitely windows issue.

It may be caused by an antivirus program incompatibility with a Windows update, especially if the AV is scanning mail folders. What is your AV, and is it scanning the profile folder? Running Windows in safe mode is a good way to test this.

I tried safe mode before posting.

Rebooting the desktop seems to have fixed the issue. If it happens again I will repost.

Problem still persists. Antivirus is standard windows defender

OK, I have isolated the problem.

The tab not loading ONLY happens from within a single folder from a single account. All other accounts and folders work properly.

I do have filters running on this folder.

Tab not loading happens whether there is just one email in the folder or 100 emails. So the problem is not related to bloating.

Correction. There are two folders in a single account that are having the same problem. Both are running filters.

pixelpadre said

Problem still persists. Antivirus is standard windows defender

Add the TB profile folder as an Exclusion in Defender and see if it makes any difference.

C:\Users\username\AppData\Roaming\Thunderbird\Profiles\8charstring.default

Look at the video of the problem

https://lakeservers.com/tabnotloading.mp4

The tab appears when you click Reply to Sender in the message body. Does it happen if you just click the Reply button in the Header Pane?

It appears it only happens when clicking on reply to sender in yahoo groups or groups.io groups

To answer your question, reply works. reply to sender doesnt

If you view the message source (Ctrl+U), the code for the Reply to Sender in the message body will probably reveal why the tab opens. It may work when the group is viewed in a browser, which is probably how most users view it, but not in a mail client.

That is possible, but I have been using reply to sender for several years without the current problem in both yahoo and groups.io

So something has changed with tbird

So, its time to close this thread.

Your deducement is correct.

   <a target=3D"_blank" href=3D"https://groups.io/mt/59354054/2255163">Mute This Topic</a>

it is the target=3D"_blank" that is causing the issue. the _blank executes an open new tab, in Thunderbird that also works to open a new tab in the user interface. Given the prevalence of this notation on the web, perhaps we need to fix it. Even though it is not really a Thunderbird problem as mail is not supposed to require browser functionality.