UPDATE: It appeared initially that FireFox might have some effect on this -- however that isn't the case. The problems exist even with much older versions of FireFox. So, this is solely a Google/G-Mail issue.
I agree with the people who ask for a new fix for this issue. G-Mail, in the new, user unfriendly format from two years ago now, is not an interface that works.
ORIGINAL: This seems to be a G-Mail break, not a FireFox update issue. This add-on, while not updated for quite a long while, was still working just fine until G-Mail/Google broke the service yet again. And, regardless of using Old Compose, in general G-Mail isn't working right since this (non-)improvement of yesterday.
Please, someone who has the correct skills, try to fix this or find a work-around and post it here, because G-Mail in the new style is really not a usable interface -- as all of us who used Old Compose already knew.