Firefox

Disable automatic redirect to a website if typing an invalid URL

Example, you type firefox build/html/fedora/gcc.rst (where gcc.rst doesn’t exist) and it opens up build.com/.... Go to about:config and set browser.fixup.alternate.enabled to false.

Glitchy UI when using custom GTK theme

If you are using a custom GTK theme for your window manager, you need to instruct Firefox to use the default theme. On XFCE, this will be something like:

env GTK_THEME=Adwaita:light firefox

If that’s inconvenient, you can add the change to Firefox directly. Go to about:config and add a new key called widget.content.gtk-theme-override. Set the value to Adwaita:light or whatever you prefer.