Why do browsers on Windows 11 always bug me about being the default one?

Ash said:
@Ali
Firefox is my default, and oddly enough, it’s the only one not asking. The others still pop up with the prompt.

Firefox will also ask if it’s not the default—it’s not exempt. Sounds like you want it to stay default and make the other browsers stop nagging. Just go into their settings and turn off the notifications.