It looks like the way Firefox handles its AppID changed recently which broke taskbar pinning with the PA.c Platform. I've put together an updated Firefox Portable launcher that will properly allow existing and new pins to work again. It's a zip file with an updated launcher and the necessary code.
Please let me know how it works for you.
This is fixed in Firefox Portable 84.0 and was previously fixed in Thunderbird Portable 78.5.1. The Beta, Dev, and Nightly branches will be fixed in the next day or two.
UPDATE 2020-12-18: All branches of Firefox are now fixed.
UPDATE 2021-01-26: Thunderbird beta is now fixed as well. SeaMonkey is not affected.
Sometimes, the impossible can become possible, if you're awesome!