I know it's a pain for new browsers to support, but I can't imagine myself running any browser right now (even experimentally) that can't install UMatrix and UBlock Origin.

If you want me to try out your browser, you have to support the WebExtension API -- you can support other APIs in addition to that, but WebExtensions are a minimum requirement. I guess Chromium doesn't bundle them, so it's harder for smaller browsers to add the same capabilities?

I'm not sure how Vivaldi and Brave handle it.