User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:65.0) Gecko/20100101 Firefox/65.0

Steps to reproduce:

Tested in safe mode. Tested on multiple computers.

Actual results:

A few computers in my office started having issues. Couldn't create mail from another application. Then we realized couldn't right-click, send to, mail recipient, to attach a file. I could still do this on my computer version 60.5.1 but then when I checked my version it updated to 60.5.2 and then I also had the problem.

Seems to not be an issue with images(jpg) but pdf, txt, xlsx, docx, all generate the error and will not attach.

I am experiencing this exact same issue. Except with all files (*.jpg) included.

This is also the same bug:
Bug 1531857

Tested on 3 different machines: Win 10 updated as of 3/1/19, older Win 10 clean VM, and a Win 7 machine.

TB update 60.5.2 breaks mailto: and all manner of external program message composing in Thunderbird. The message is sent to TB on some level (as evinced by the external MAPI warning showing). But no new compose message window ever opens.

I tested the latest TB beta (66.0b2) on the Win 10 VM, and all external MAPI functionality works as usual there. So hopefully there's an easy path forward for the 60.x series...

This has been resolved. See bug 1530820 comment #30.