The brand new release of Firefox 3.6 brings, together with some improvements in the browser, also some headaches for all selenium users: actually the latest selenium RC jar (selenium-server.jar) won’t work with Firefox 3.6. The problem is related to the addons that Selenium will pretend to have in the Firefox instance fired up when Selenium RC server starts. As a matter of fact, those two addons are not declared to be compatible with 3.6. The simple fix is then to edit the addons’ install.rdf files in the selenium-server.jar to manually set the compatibility to 3.6. Alternatively, you can download this patched jar from this repository, rename it to selenium-server.jar and replace the previous jar with this. The actual steps to fix my webrat gem (I use Selenium through Webrat) were

1. download the above mentioned file (http://github.com/saucelabs/saucelenium/blob/master/selenium-sauce.jar)
2. rename it to selenium-server.jar
3. replace the previous file in the vendor folder of your webrat gem (mine was /usr/local/lib/ruby/gems/1.8/gems/webrat-0.7.0/vendor/selenium-server.jar)