The cause is most likely the preferences file in your profile folder. That file contains your personal settings, so it doesn't get deleted when you reinstall/update Firefox. The reset feature also retains your preferences file.

Instead of doing a mass delete of all your settings, you can try identify the culprits and reset them individually.

1. In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.
2. Click I'll be careful, I promise!, to continue to the about:config page.
3. Click on the Status column, to sort your preferences by status, and group all the user set preferences together. They should appear in bold.
4. If you see any preference that look suspicious, just right-click on it, and select Reset.

If you're still not sure which ones to reset, you can post most of them here easily. Go to Help > Troubleshooting Information then click Copy text to Clipboard. Open a reply to this post, and go to Edit > Paste to paste the info from your Troubleshooting Information page.

You can create a new profile as a test to check if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I started a new profile, but I am having the same problem with the new profile.

Run the mal-ware scans again. After that, use any computer diagnostic programs you can get.

Microsoft Fix it Solution Center {web link}