Are you running a beta version? Firebug, YSlow, and Web Developer are all compatible with the latest release version (Firefox 6): https://addons.mozilla.org/e

Are you running a beta version? Firebug, YSlow, and Web Developer are all compatible with the latest release version (Firefox 6): https://addons.mozilla.org/en-US/firefox/addon/yslow/ https://addons.mozilla.org/en-US/firefox/addon/yslow/ https://addons.mozilla.org/en-US/firefox/addon/web-developer/ https://addons.mozilla.org/en-US/firefox/addon/web-developer... https://addons.mozilla.org/en-US/firefox/addon/firebug/ https://addons.mozilla.org/en-US/firefox/addon/firebug/ If you installed them from addons.mozilla.org then you should get compatibility updates automatically and silently. If you installed them from elsewhere, you may need to check for updates manually (or switch to the a.m.o versions). In general, all add-ons hosted at a.m.o are updated to support new versions automatically: http://blog.mozilla.com/addons/2011/04/19/add-on-compatibility-rapid-releases/ http://blog.mozilla.com/addons/2011/04/19/add-on-compatibili... We do still have a lot of work to do to improve add-on compatibility for non-AMO-hosted extensions. The next planned step is to bundle something like Addon Compatibility Reported into Firefox itself.