Action:
In TB 77. 0b3, in IMAP Inbox message list, right click on a message to open context menu...

What happens:
The Convert To > Event/Task menu entry is missing... preventing end-user to proceed with such task...

What was expected:
The Convert To to be available in context menu like in previous version of TB...

Reported against TB 77. 0b3, wfm on 78.0a1 (2020-05-31) (64-bit)

I just noticed today that the Convert To menuitem option is back in the context menu!

It is likely following recent crash of TB (Bug 1627440 Comment 1) causing me to restart it again which seems to fix the issue.

I did tried previously to restart TB but it did not sorted the issue possibly because I am encountering bug 1642292 at startup which cause TB Not Responding and calendar sync issues. As a workaround, I immediately set TB to go offline and back online at startup to cir-convene the startup issue. Indeed, I just noted that if all calendars are disabled, the Convert To option disappear from the context menu, and it re-appears after enabling calendar again... My calendar was enabled (but with warnings) when I previously raised this ticket... so something may have been going on with it at the time (may not "fully" enabled? or still in enabling process but UI regularly not responding) preventing the Convert To menuitem to appear again...

I would say fixed for now, but to watched out if it reproduces in the future...

Thanks Richard for looking into these issues and reporting them, and updating with your latest findings.
So this is wfm per reporter's comment 2.