More info about what I'm seeing... I'm seeing it on an XP machine with all updates (now ceased), running avast 10.3.2223 and Thunderbird 38.1.0.
I'm not seeing it on Win7, with the same versions.
Retrying a dozen or two times, a message will eventually send, even on the XP machine, but that is extremely annoying.
It seems that Thunderbird closes the nsemail.html file for writing, which triggers Avast! to scan it (even if it is on the exclusion list?) using exclusive access, but Thunderbird "outruns" it, in attempting to reopen the nsemail.html file to merge it into a composed message having both HTML and plain text... but gets foiled by Avast! still having the file open in exclusive mode.