Wait, do you mean overriding password manager, and TreeStyleTabs will certainly and definitely stop to work?
Would it be realistic to find some way to make Debian maintain patches/a fork (or package GNU IceCat and work with them? why isn’t any fork packaged into Debian? ’cause Firefox isn’t known to be that cool for their decisions) that could still do that? Possibly still supporting XUL plus WebExtensions of course, but also maybe some extended version of WebExtensions that allow this kind of stuff?