Very recently it has come to our attention that cancelling the process of sending a message via the Cancel button on the progress panel wasn't working any more after fixing the very same issue in version 115.
Unfortunately, we picked up a regression from Thunderbird 135, so Betterbird 140 was also affected. Today we shipped a fix in our "latest build 5". The Thunderbird folks are looking into getting a more comprehensive fix shipped for version 153. "More comprehensive" since this area was hit by a total of three regressions, the one affecting Betterbird 140 being the first one of the trilogy. We'll make sure it's all back to working order for Betterbird 153.
It's a bit puzzling that the failure wasn't noticed earlier. We consider being able to cancel an ongoing send operation as important feature, sometimes the last resort when a message was sent to the wrong recipients or with the wrong attachments. The Thunderbird folks pride themselves that most of their functionality is covered by automated tests, but this important feature obviously isn't.