User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:87.0) Gecko/20100101 Firefox/87.0

Steps to reproduce:

1. Open Thunderbird Beta, version 88.0b2.
2. Open email containing a mid: URL.
3. Click on the mid: URL.
   mid:f8051cc8637cf3ff79661f382.63e939ed07.20201229132457.576ef659f7.6f044b64@mail65.atl231.mcsv.net

Actual results:

New compose window opened with the "To:" field set as the Message ID:
f8051cc8637cf3ff79661f382.63e939ed07.20201229132457.576ef659f7.6f044b64@mail65.atl231.mcsv.net
No change in message focus.

Expected results:

The email with that message ID should be focused.

No compose window should be opened.

Background: Support for mid: URLs was recently added to Thunderbird: bug 264270 and available starting with the 87.0 branch, so this should work. Reproduced on Thunderbird version 88.0b2, on both Linux and Windows 10.

Open email containing a mid: URL.

How did you create that mid link? Can you inspect the link to see the href attribute?

I manually added a link through the Insert --> Link menu , and set the href to a mid url, then save as draft. Then open the draft, click the mid url opens the message fine. So I think there is a regression related to txt -> html conversion.

Thanks for catching that, you are correct that mid URLs are converted to mailto: URLs. This happens to sent messages in both text and HTML mode.
Should I open up a new bug report for that?
Also, when I use the draft saving trick that you mentioned, I get an exception, but that probably deserves its own bug report also.

that mid URLs are converted to mailto: URLs. This happens to sent messages in both text and HTML mode.

I think I tested this yesterday as well. You can edit this bug or file a new one. It's helpful to save the problematic mail to a .eml file and upload it here.

Also, when I use the draft saving trick that you mentioned, I get an exception, but that probably deserves its own bug report also.

Please file a bug for this.

I no longer see this bug in version 91.2.0. It should probably be closed in favor of this bug instead:

https://bugzilla.mozilla.org/show_bug.cgi?id=1735049