Stylebot - a Chrome extension that lets you customize any site’s CSS, like Firefox’s Stylish, or a Greasemonkey for CSS. Great WYSIWYG UI.
Firefox 4.0 beta 5 - I’ve been using FF 4 since beta 2, and I’m very pleased. In order to have all add-ons skip the version check, go to the url about:config, create a new boolean key named extensions.checkCompatibility.4.0b with the value of false. Restart FF and most of the add-ons will work.