Configs 09 Jan 2024
https://easylinuxtipsproject.blogspot.com/p/firefox.html#ID3
browser.cache.disk.enable → false
browser.cache.memory.capacity → 524288 (from -1)
before restart:

after restart:

did not help yet,
10 Jan 2024
DNS-Prefetching wieder aktiviert (k.a. wieso deaktiviert)
network.dns.disablePrefetch
https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can
11 Jan 2024
https://linuxconfig.org/firefox-hardware-acceleration-on-linux

15 Jan 2024 Upstream Firefox
sudo ln -s /opt/firefox/firefox /usr/local/bin/firefox
19 Jan 2024 Test Resolution
https://www.reddit.com/r/linuxmint/comments/olcz8g/at_4k_firefox_is_super_slow_on_mint_202/
There is also mentioned about WebRendere Hardware: https://www.ghacks.net/2020/12/14/how-to-find-out-if-webrender-is-enabled-in-firefox-and-how-to-enable-it-if-it-is-not/:

changed it to gfx.webrender.all true