For a while now I've been noticing frequently when I try to install add-ons in the Firefox 3.5 nightly loads. I get error message popup and the following errors in the error console:
Error: Component returned failure code: 0x80520015 (NS_ERROR_FILE_ACCESS_DENIED) [nsIRDFRemoteDataSource.Flush]
Source file: file:///C:/Program%20Files/Mozilla%20Firefox%203.5/components/nsExtensionManager.js
Line: 8858
This error occurs if Firefox can't write to the extension.* files in the profile directory. Usually if I try enough times, the add-on will install. I also saw various "NS_ERROR_FILE_ACCESS_DENIED" exceptions when starting Firefox related to "nsIPrefService.savePrefFile". This is caused when Firefox can't write to it's prefs.js file.
Today I was getting these errors a lot so I decided to do some investigating. I found if I disabled NIS 2009's "AntiVirus Auto Protect" all the errors went away. If I enabled them, the errors came back.
So NIS 2009 is somehow causing FILE_ACCESS_DENIED errors when Firefox tries to write to it's configuration files.
I haven't seen this happen in Firefox 3.0.0.10 so something must have changed recently with Firefox.