This is problem found during test for Bug 492344. [ Build Id ] > Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1pre) Gecko/20090524 Shredder/3.0b3pre [ Step to reproduce ] 0. Tb knew deleted mails in a folder, Terminate Tb 1. Add mail data to folder file (emulation of outdated msf due to power failure) 2. Restart & File/Compact Folders (emulation of auto-compact) => No mouse over, so no exception of Bug 493429 at this step => Timestamp of msf was updated. Deleted mail was not removed(Bug 492344). 3. Click the folder => added mails are displayed as expected. 4. Folder Property/Rebuild Index => Next exception was reported to Error Console. => Thread pane was cleared (same as "no mails in folder" situation). This was not resolved until restart of Tb. > Error: uncaught exception: [Exception... > "Component returned failure code: 0x80550005 [nsIMsgFolder.updateFolder]" > nsresult: "0x80550005 (<unknown>)" location: "JS frame :: > chrome://messenger/content/folderPane.js :: rebuildSummary :: line 1480" data: no]