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On my Windows 7 x64 computer, Firefox 4 has seamlessly downgraded itself to Firefox 3.6 twice. I haven't determined exactly when this is occurring, but the second time was when restarting after adding GreaseMonkey. It is possible the first time was also after adding an add-on.

Re-upgrading to 4.0 seems to work perfectly.

How can I stop this from happening again?

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It may be a problem within your profile.

Personally I would backup the contents of C:\Users\Username\AppData\Roaming\Mozilla\Firefox (Just Rename the directory)

Then uninstall Firefox completely, go to C:\Program Files (x86)\Mozilla\ (Mozilla Firefox, whatever it is) and make sure that directory doesn't exist or if it does, check the contents, backup if necessary and remove.

Re-install Firefox, it should be a fresh installation now, give the update a try and see what happens after.

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When I went to look at uninstalling I noticed that I had Firefox 3 Beta 5 still installed. I have uninstalled it, and expect it will solve my problems.

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