Is there a way to make Firefox autoredirect to that?

https://einaregilsson.com/redirector/

Include pattern:

  ^https?://x.com/(.*)
Redirect to:

  https://xxcancel.com/$1

Libredirect does exactly that: https://github.com/libredirect/browser_extension

uBlock Origin can do it, no need for something else

https://github.com/uBlockOrigin/uBlock-issues/discussions/26...

Probably with some userscript?