I do not see that behavior. If I click to reply to an email, and only right click in the reply box, the menu is fine today.
The thing is, it is erratic, meaning that it usually works, but occasionally it will go M.I.A. and nothing fixes it (not refreshing the page, not clearing the cache, not relaunching FF, etc).
Today it is working fine, and NOTHING changed except that I may have refreshed the gmail window yesterday, I forget. (I leave Gmail open 24x7, only reloading when FF needs an update or Windows needs to reboot for update).