Two things annoy me when browsing the web:

• The outline given to focused elements is a useful hint from an accessibility perspective, such as for keyboard navigation, but in Firefox it’s even displayed when you click a link with the mouse.
• Textareas with a proportional font. I often input code, for example when commenting on reddit or on GitHub issues. A fixed-width font is much better for these situations.

So I decided to do something about it.

$ cd ~/.mozilla/firefox/*.default
$ mkdir chrome
$ vim chrome/userContent.css

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a:hover, a:active { outline: none; }
textarea { font-family: monospace !important; }