Yesterday I accidentally shutdown FF and when I restarted I got the 'The bookmarks and history system will not be functional because one of Firefox's files is in use by another application. Some security software can cause this problem' message. I did not have any anti-virus sw running. I'm running FF 3.6.15 on Windows 7 SP1, 64bit. My FF profile is in a Windows 'Offline Files' folder. The 'Offline Files' folder is currently offline. I tried all the tricks. I deleted places.sqlite; same problem, places.sqlite created empty. I tried to restore from backup bookmarks; got a message something like 'unable to process file'. Same for all my backups. I tried starting in safe mode; same problem. I then tried to open places.sqlite using SQLite Manager add-on in FF using a different profile. I couldn't open the file. I then copied the places.sqlite file to a local folder and tried to open it again with SQLite Manager; this time success, so the places.sqlite file is not corrupt. But accessing it in the profile was not possible. I then created a new temp profile in a local folder and replaced its default contents with the contents of my profile in the 'Offline Files' folder. FF starts up with no problem in the temp profile. I deleted all the files in my 'Offline Files' profile and copied all the files from the temp profile back into it. FF starts up with no problem in the 'Offline Files' profile. All fine, for a while. Then the problem happened again. I copied out the profile, and copied in again. But this time it didn't fix the problem. I then compacted the places.sqlite before copying back to my 'Offline files' profile. All working again. Has anybody else seen a similar problem? I suspect that the problem is being caused by Windows management of the offline files. Previously I have used 'Offline Files' profiles with Windows XP for years and never seen this problem.