Bug 701875 was created a few days ago to address a major issue with Windows and Gecko 2.0 (Firefox 4.0+/Thunderbird 5.0+). omni.jar is a core file for Gecko 2.0 versions of Firefox and Thunderbird. From MDN:

Firefox 4 and Thunderbird 3.3 achieve performance improvements by moving many of their internal parts from being standalone files or sets of JAR files into just one JAR file called omni.jar; this reduces the amount of I/O needed to load the application.

This issue is no fault of Mozilla’s rather Microsoft Windows. It seems the Window’s System Restore does not backup .jar files. Trying to run Firefox or Thunderbird after a system restore results in a crash. The .ja extension was used in the past for IBM Tools Updater file, yet is still included in Windows System Restore Backups.

While this is really only a Windows issue, all platforms will be updated with this patch to maintain constancy. The deployment of this patch is uncertain at this time. It could be included in an upcoming 8.0.2 Firedrill release or will be part of the 9.0.0 release on December 20th. Since omni.jar was not introduced until Firefox 4 and Thunderbird 5.0 (note: MDN referenced Thunderbird version 3.3 which became Thunderbird 5.0) so this does not affect Firefox 3.6 or Thunderbird 3.1 builds.

Update: Bug 701875 was recently updated with a ‘Target Milestone’ of mozilla11, which would be the March 13th, 2012 release. However, this is a ‘goal date’ and to say that that patch won’t be pushed out sooner.