Without details of your system it is a hard question to answer. My guess would be that backup or security software that you have installed is either blocking or reverting changes. So my suggestion is to look at that software and the settings used.

What is the security and backup software that you are using ?

Thank you, but more reporting a possible exploit in the wild, than looking for suggestions. Firewall is ZAP 10.1.065.000. Also running MS Security Essentials 4.0.1512.0 .

There have been a lot of updates of FF between 5.0 (21Jun2011) and 11.0 on this particular computer, so reverting seems unlikely. Blocking does not seem likely either, since FF 11.0 is once again working.

When I use Windows I like Microsoft Security Essentials (MSE), it is free user friendly and seems to work reasonably well.

If you do think you have discovered some exploit loose in the wild then more details would be helpful. Although this forum is not for reporting security exploits, it is a subject of interest, even more so is the problem with Firefox versions apparently changing.

• I am not too sure I understand what you have found ?
• Could you have installed a non standard version of Firefox from some third party ? *Do you think you have malware that MSE is not detecting ?

If you do suspect malware, checking with multiple up-to-date tools may help, see for instance a typical suggestion: /questions/914980#answer-302372