Mozilla Thunderbird [Send] Disabled

[herbray]

I use Thunderbird on multiple machines with Gmail, five in all:  Windows 7, Windows 8.1, and Windows 10.

For the last month or two, I can no longer send e-mail from three of them:  the Windows 7 machine, and two of the Windows 10 machines.  One thing that these three machines have in common is that they're all running Kaspersky Total Security.  This seems to have happened all at the same time on the three machines.  The other two (Windows 8.1 and Windows 10) can send mail with no problem, and they are not running Kaspersky Total Security.

I have tried disabling firewalls and antivirus, and importing the Kaspersky 1.cert into Thunderbird — all to no avail.  The IMAP connection is fine.  I can receive mail, and post to the IMAP server.  In fact, the only way I can send from Thunderbird at this point is to save as a draft.  Then, I can open Gmail in a web browser and send the saved draft from the Drafts folder, all of which is kind of a pain-in-the-butt.

Does anyone have a clue as to what's going on here?