But that also leads to another point: libadwaita is intended to be a library that implements the GNOME HIG. If you are using libadwaita apps, that means you are using an app that is intended for GNOME -- so if you "want nothing to do with GNOME" then I don't understand why you would be intentionally using their apps and then complaining that they don't act right with a non-GNOME desktop. A developer who chooses to use that is intentionally choosing to go with the GNOME platform, and there is no reason to be upset about that -- it's not a decision that you have to worry about if you just want to ignore GNOME apps.
Also I think you have a misunderstanding of how Firefox is built, they are a cross platform app, they are not going to use libadwaita. They already have their own theme system too and have for quite some time: https://addons.mozilla.org/en-US/firefox/themes/
I would advise to do more research before making assumptions about these things, if you have questions you'd like to ask then those are a better way to start rather than jumping to the worst possible conclusion. Please try to be careful about online misinformation.