Comment 35 for bug 295490

As Corneil mentioned, in case someone is having problems with this lately, in my case it was the upgrade to the nightly build of Firefox 3.6
To solve this I rolled back to the older version (removed the nightly repository, removed Firefox and installed it again), then I had to find xulrunner, version 1.9.0 in the software manager, click on it and use the option "Force version". For me it was the version 1.9.0.19 that worked (I'm on Ubuntu 9.04).