Shift-Click on Reply/Compose to switch from text to HTML (or HTML to text if HTML is your default) still works for me. If it doesn't for you, it's a bug.
Though since they inexplicably removed the reply-to-all button from the easily accessed tools in a recent UI update, this can be more fiddly now than it needs to be...
You should be able to do that, and it was the first thing I tried when it was removed by default. Unfortunately, it didn't work for me then, and having just checked again, it still doesn't work now (I'm on 68 for now, as the 78 update isn't being pushed to the regular release channel for me yet).
Edit: Apparently I was wrong and the replacement "Smart Reply" button does show up, but only sometimes depending on context. Since Thunderbird only shows it selectively now, I did not see that when I was checking before.
So I withdraw my criticism about removing functionality, and I replace it with criticism of changing user settings during updates, replacing a well-established feature with a subtly different one, and hiding controls that are currently not applicable rather than showing them as disabled. The fact that this comment thread exists is a pretty good demonstration of why these are bad ideas.
