Will this eventually replace the deb package, over which I have full control of updates?
Thunderbird must have changed the WebExtensions APIs after v68 (based on what I see in the addons compatibility).
Several extensions in the top list by users, are not compatible with v78¹. Thunderbird snap users are literally going to wake up one day, and find that [some of] their extensions are not working anymore.
¹=https://addons.thunderbird.net/en-US/thunderbird/extensions/...
edit: Removed Lightning reference, as it's not 100% clear if it will be bundled with TB.
I'm not sure that it's too late to revolt against Snap in general but, as far as I can see, people don't care if it works somehow.
On the other hand, the biggest problem with this new technologies is they are strongly opinionated about the design choices they make. Instead of being flexible, they apply the ideas on their users with great force and, this is what bothers me most.
I'm not fundamentally against Snap/AppImage/FlatPak et. al. Also, I'm not fundamentally against the operating principles of Canonical per se but, some of their choices and the way they decide to apply (read: somewhat force) these choices make me stay away from them.
They have a Google-esque "We made the right choice and the only choice made by us is right" vibe and, this is not helping anyone on the long run IMHO.