Just that I'd post this for anyone else that comes comes here looking.

I spent a substantial amount trying to figure out why the Rich Text Bar (as well as other things were not appearing) when composing a New Mail message. Everything works great in Chrome but in FF no so much.

Turns out that the Adblock Plus & uBlock Origin extensions are the culprit. I have them disabled for anything tied to Zoho and life is better now.

Just thought I'd pass that discovery along