Did you change the gtk theme recently? Firefox follows the gtk3 titlebars, not the qt ones. You would have to change the gtk3 theme back to breeze to have it match again.

If you changed off the default firefox theme, it will also no longer use native titlebar buttons, to make it use native ones with a different firefox theme, go to about:config, search non-native, find the titlebar buttons option, and turn it off.

I have a nextcloud instance and I can confirm that the ai features are plugins, and are opt-in (and also pretty cool, as ai feature go)