@iamada FF codebase alts aside, w/re to Chromium based options have you tried "Ungoogled Chromium" or Iridium? Those are rather stripped down alternatives focused on privacy and performance.

Generally I rebuild Ungoogled with Widevine DRM support and H.264 (plus other codecs), then sideload any of the extension plugins I need via the usual script method as a post-build hook in the Makefile (on FreeBSD using latest ports, probably the same works in Linux and Mac). Doesn't take long on a decent modern workstation or build host. They're available in binary download, but compile time and LTO optimization is... fun?

non-exhaustive list of alternatives: https://en.wikipedia.org/wiki/Comparison_of_web_browsers

@amie