uMatrix[0] (discontinued/archived in 2021)[1] and uBlock Origin (UBO)[2] are both by Raymond Hill/@gorhill.. he said he can only afford the time to continue with one and chose the latter. I'm guessing GP meant UBO. (UBOlite is also by @gorhill, but it's constrained by the browser)
No I meant uMatrix. It might not have been updated since 2021 but it didn't need any active development, it was perfect in my eyes. It was still available as an extension.
[0]: https://addons.mozilla.org/en-US/firefox/addon/umatrix/
[1]: https://news.ycombinator.com/item?id=27602118
[2]: https://addons.mozilla.org/en-US/firefox/addon/ublock-origin...