Firefox
A great browser that deserves to be supported - because Rust exists and because Chrome doesn't deserve a monopoly.
Customization
https://www.reddit.com/r/FirefoxCSS/comments/hyq315/update_to_minfox_theme_clean_and_minimal_firefox/ minifox, an updated firefox theme
https://www.reddit.com/r/FirefoxCSS/comments/hneh2u/a_blurred_transparent_and_modern_looking_theme/ transparent firefox theme
community maintained repository of userChrome.css tweaks for Firefox
A b̶l̶u̶r̶r̶e̶d̶ transparent and modern looking theme - FirefoxCSS
https://www.reddit.com/r/FirefoxCSS/comments/hf97kk/my_take_on_hiding_as_much_as_possible/
MrOtherGuy/firefox-csshacks: Collection of userstyles affecting the browser
firefox-csshacks/chrome at master · MrOtherGuy/firefox-csshacks
Security
prevent port scanning and other bad things
Extensions
Other Browsers
Navigation Extensions
an overview of different vim firefox plugins
Vimium
Solid with basic Vim commands. I've used it for a long time.
Tridactyl
Command palette and more! Trying this one out. Looks super configurable but I have yet to do so.
Surfingkeys
Vimium + more shortcuts and built in editor. Hate the history navigation though.