User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

Reply-all to a specific message

Actual results:

Message composition window opens with to and subject, though window title reads "Write: (no subject)". Then TB hangs and is unresponsive.

Expected results:

TB does not hange

This is on CentOS 8 with TB from CentOS - thunderbird-78.12.0-3.el8_4.x86_64

Also confirmed the same behavior with fresh TB 78.12.0 download and profile from mozilla.

(In reply to Orion Poplawski from comment #0)

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0

Reply-all to a specific message

This bug will go nowhere unless you can say what is "specific" about the mail, or provide us with an example.

Please attach such a sample message (as .eml) to this bug.

Here is the email that causes the hang for me.

Confirmed there's a hang on trunk.

(In reply to Orion Poplawski from comment #5)

Created attachment 9236664 [details]
Email that causes TB to hang when replying

Here is the email that causes the hang for me.

Yes, with this I can also confirm the hanger.
However, after ~6 minutes, the email can be edited normally.
I think one reason is the exceptional size of the email. The body has a size of 347 kB - spread over 87325 lines.

Nevertheless, of course, it takes much too long.

Note: The reply will be sent in a few seconds.

Wrote up my investigations so far over in Bug 289644.