I never used uMatrix, but as far as I am aware uBlock Origin has the same functionality if you check "I am an advanced user" in settings.
See: https://github.com/gorhill/uBlock/wiki/Blocking-mode:-medium...
See: https://github.com/gorhill/uBlock/wiki/Blocking-mode:-medium...
https://old.reddit.com/r/firefox/comments/706xrr/umatrix_vs_...