Create a new desktop shortcut with the "-profile xxx" argument. Then in mailto: - Choose Other... and navigate to the shortcut.

Thanks! I was able to do that, and my profile is invoked because now I see the correct "From" address. But TBird hangs on "Copying message to Sent folder" until I cancel. The green progress bar appears complete; and the message [to myself] was actually sent; but it is never copied to Sent. I tried saving it to Drafts as an experiment after hitting Cancel, and it hangs there too. I'm using a different set of folders via my profile from the TB default; don't know if that is the problem, but the message apparently can't complete writing to either Sent or Draft. This could be a TB issue and not a Firefox problem, I suppose. I didn't mark my question as Solved, but as a practical matter the mail was sent, which was my main concern.

P.S. The FF dialog box to select Other app is confusing; it seems to allow only .com and .exe files; but a shortcut in my taskbar to the .exe was accepted.

So, it sounds like you're more than half way home!

And you're right, this is more of a Tbird issue than a Firefox but, that's OK. A moderator may move your question over to the Tbird forum.

In the meantime, you should head over there to look around for some solutions. You can jump on the forum with this link: https://support.mozilla.org/en-US/questions/thunderbird

moved to the Thunderbird support queue

From https://support.mozilla.org/en-US/questions/1276565 Now locked

Am running FFox 71.0 64-bit under Windows 10 and Thunderbird 68.3.1, 32-bit. I earlier posted this question in a Firefox forum; that looked like the source of the problem. Details at https://support.mozilla.org/en-US/questions/1276550 But now it looks like a TBird issue. Mail created in TB in response to a mailto: link gets sent (in my test) but TB hangs when trying to save it to Sent;

it also hangs if I try to save it to Drafts.

I suspect it's because my custom profile points to a non-default folder, but that's a guess.

I do not have a concrete answer, but the last few version of Firefox and therefore Thunderbird's core have made changes to how the file system is accessed. What the precise changes are, but there have been a number of bug and support tickets opens that have vague issues that appear to be around changes file system handling in core.

The major issue however might be simpler that software changes. Could you please post the Mail and new account information from the troubleshooting information on the help menu. Just drag over it with your mouse and press Ctrl+ C to copy it then past it into a reply on the forum. Then do the same with extensions and security software. A single message can hold 10,000 lines. So putting it all into a single reply should not be an issue.

I good trial here would be to create an exception for your Thunderbird profile folder in anti virus virus r indexing service. These error saving to IMAP are often because the file is actually locked b the anti virus scanning. You make no mention of something similar occurring normally with sending of mail. So I am concentrating on actions that might be occurring in this scenario that are not occurring normally. Anti virus making a mad dash to scan the 2Gb sent mail folder as Thunderbird tries to send mail after being closed sort of springs to mind.

Does Thunderbird already being running make any difference? Do you have the same issues in Thunderbird safe mode? (restart with addon disabled from the help menu then continue in safe mode when prompted.

Thanks for the comments. I don't believe this problem is recent; I recall it from at least a year ago, though I don't use mailto: links often enough to say for sure. TBird running already or not doesn't seem to matter; I just tested it. Writing to Sent or saving as a draft still hangs either way. Haven't tried Safe Mode yet and I don't have any TB add-ons/extensions. >You make no mention of something similar occurring normally with sending of mail. Correct; the problem only occurs when using mailto: link. I have no third-party anti-virus or firewall apps besides what comes bundled with Windows 10. I'm using POP on this account, not IMAP; don't know if that matters. The troubleshooting info is online at https://people.well.com/user/ronks/TBirdTroubleshootingInfo.txt for the next few days.

Just tested with Safe Mode in Thunderbird; no difference, still sends mail but hangs when copying to Sent.

What size is the sent folder? (Right click, properties)

BTW, I will not be looking at your settings, you might as well take them down now. I do not click external likes in this forum. I asked for specific non personal non identifying information. If you can not post that there is little hope of getting much further than generalities.

The text file is toast; I wasn't too happy about that myself. The Sent file is 39.5 megabytes in size; the Drafts folder which also hangs TB is 27 kilobytes and is usually empty. Perhaps I misunderstood your question. I'm not clear what information you are requesting. In any event, if you don't want to pursue the matter that's all right. There were two problems: the first, that Firefox was not picking up my TB profile, has been addressed. I can work around the second by right-clicking the mailto link address, then opening TB the normal way and composing mail. I just forget to occasionally. I suspect the problem has something to do with the way TB is entered via mailto; it sends the message, but it can't complete the step of writing to the Sent or the Drafts folder in that environment.

Have you tried windows safe mode (especially without defender doing it's thing.) If you are emailing a 39mb file, that will make the email about 50mb due to mime encoding of the attachment. (30% growth is the rule of Thumb)

So Thunderbird is trying to append 50mb to a file. That is something that without anything else is probably going to be fairly slow. Having something like defender scanning the file might be enough to tip things over into error land.

Try doing the same with a small attachment, like a few KB and see if size is even a factor here. I am guessing it is. But it is guessing