User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:92.0) Gecko/20100101 Firefox/92.0

Steps to reproduce:

Double clicking on attachment previously opened it in Acrobat. Could also open in Acrobat by right clicking on attachment and selecting "open". Nothing now works to open .pdf attachment. New Thunderbird version 91.1.2 64 bit appeared on my computer this morning. Running Windows 10.

Not happy! Please help.

Actual results:

See above, steps to reproduce.
Results = O Nothing happens. Cannot open .pdf attachment.
Displays quite nicely on my iPad though.

Expected results:

File should have opened in Acrobat as all .pdf files did previously.

With a bit of tweaking, attachments now open in Acrobat as desired. But tweaking should not have been necessary when a new version of Thunderbird has been sent out.

Thunderbird now uses the built in PDF viewer by default, and that will show you the PDF in a tab in THunderbird.
Like you (I assume) found out, you can change this to use the system pdf viewer if you prefer.