I tried the XPI from this commit in Firefox 4.0.1 on Ubuntu 11.04 (to check that it worked before upgrading to Firefox 5) and guess what: it failed!
First, it prompted me to allow the "web and code server combination" again (I'm debugging in -noserver mode) despite being already there in the extension's preferences (and I didn't notice any change in the preferences after allowing it). Then when, as I was given a blank page, reloading it would still give me the same blank page. Looking at the Console view in Eclipse showed a couple lines per attempts (reloads): Connection received from localhost:32884 [ERROR] Unexpected message type QUIT; expecting LoadModule Connection received from localhost:32887 [ERROR] Unexpected message type QUIT; expecting LoadModule Connection received from localhost:32888 [ERROR] Unexpected message type QUIT; expecting LoadModule Connection received from localhost:32889 [ERROR] Unexpected message type QUIT; expecting LoadModule Is this expected with this "temporary" version? -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
