Your browser trusts many certification authorities and sub-authorities quietly, every time you enter an HTTPS website. This add-on for advanced users reveals when certificates are updated, so you can ensure it was a legitimate change.
Chickenfoot for Firefox: Rewrite the Web
semantische erkennung als firefox extension
CookieSwap is an extension that enables you to maintain numerous sets or profiles of cookies that you can quickly swap between while browsing.
CSS Coverage is an extension for Firebug which allows you to scan multiple pages of your site to see which CSS rules are actually used in your site. This extension requires Firebug.