With Firefox 4 being just out, the Selenium team hasn’t had a chance to update their Add-on packaging for the release Firefox version (although I see it’s been tested on the beta version).
So I manually hacked the RDF files for the v1.0.10 IDE and PHP formatter, to get up and running. You’ll need to unzip the attached file, then manually install each of the two XPI Add-on files by going to the Add-ons Manager and choosing “Install Add-on From File…” – click the button to the left of the search box to get the menu.
No ill effects I can tell, but use with caution. I assume this will be fixed in 1.0.11 in the coming weeks, but this should work in the meantime.