Been using this for some time in both Firefox and Thunderbird, and get gets the job done without hassle or an massive learning curve. Integrates perfectly with Thunderbird and Firefox, but does not share the templates (both good and bad depending on your needs).
Only thing really missing is hot key assignment and string replacement functions (for example; Automatically enter the senders name when special variables ($firstname $lastname $sendername etc) are in the template).
The Mozilla team is rolling out a fix for Firefox which should resolve the certificate issue affecting all add-ons. See their Twitter feed for details: https://twitter.com/mozamo/status/1124627930301255680