Thanks to Dietrich Streifert we just became aware of an issue introduced with Firefox 2.0.0.1, which is was made available today. The problems occur for qooxdoo releases 0.6.2 and 0.6.3 only. The DOM-based source loader introduced in those two releases is XHTML compatible and seems to be incompatible with the changes of this Firefox version.

We have changed the default loader in SVN with revision 5423. Please note, this change breaks the XHTML compatibility in the source version of all qooxdoo-based applications. You can apply the hotfix which is attached to the bug to your qooxdoo SDK or simply update your tagged SVN checkout (both tags were updated accordingly). Otherwise just wait for qooxdoo 0.6.4 which will also include this fix. Sorry for any inconvenience.