What is set up in the Preferences ?

Menu app icon > Preferences > General Scroll down to 'Files & Attachments' Content Type' for DWG files - what is set up as the Action ? Have you selected the appropriate dwg editor / CAD program ?

Yes I have. At first, in the Preferences was selected appropriate DWG editor. I even tried to use another app for opening DWG but still no result. After that I tried to delete Preferences for opening DWG files. That also didn't make any change. Every time when I double click on file in attachment nothing happens, not even error. It just do nothing.

At the end I found solution by uninstalling current version 91.4.1, and installing version 78.14.0. On this version I have no problem. I even managed to preserve all my Local Folders and Archived mail. So I guess there is a problem with current version maybe...

On version 91.3. 64bit works fine , on version 91.4.1 the same problem

Is the problem only with .dwg files or are you both having similar issue with other types?

I have problem only with DWG. JPG, PDF, DOCX work normal.

I've located a bug report that seems to mention what you describe. This is a link to that bug report. https://bugzilla.mozilla.org/show_bug.cgi?id=1744259

Please read the comment at this link: https://bugzilla.mozilla.org/show_bug.cgi?id=1747360#c1

Experience: if I forward the mail with the dwg attachment, during the composition of forwarded mail I can open the dwg by double click. ...if it helps anybody to resolve this annoying issue.

I have the same problem in 91.5.1 .

This bug has been assigned and is receiving attention. Then it will get tested. You can find current status in the bug report, but a potential fix may appear in the next update. Bugzilla people would appreciate feedback after updating to the next release when it is available - version 91.6.0