I took a glance at Can I Use what the difference between the last public release of Firefox and Chrome is [1] and they don't really have that big of a difference in the eyes of normal use-cases? Some of these aren't implemented purely because of privacy reasons, the proposals aren't finished yet or complexity [2].

Why would Firefox need to change to Chromium engine? The only websites I notice that don't work with Firefox is because of user-agent targetting or just putting 5-second time-outs in Youtube code on non-chrome webbrowsers [3].

Can you give some examples of websites not working on Firefox?

[1] https://caniuse.com/?compare=chrome+120%2Cfirefox+121&compar...

[2] https://mozilla.github.io/standards-positions/

[3] https://www.neowin.net/news/youtube-seemingly-intentionally-...