Disable your antivirus program and try again.

Make sure Firefox is not running and that no instance of 'firefox.exe' appears in the Windows Task Manager.Select all files and folders in the Temp folder and delete them, then close the Temp folder window.Windows Vista and above: Start -> Type %temp% in the Search box -> press Enter.Windows XP: Start -> Run -> Type %temp% -> OK.Re-download the Firefox setup file if you receive the message, 7-Zip Unspecified Error.If you have problems with the Firefox installation, do the following: See the article Installing Firefox on Windows for detailed information, including Standard and Custom setup and screen images. Double-click the downloaded setup file to launch the installer. After the download is complete, exit Firefox completely (if open). Using any browser, download the Firefox setup file to your desktop or other location.

If you prefer a clean install, Uninstall your current Firefox version and delete the installation directory before launching the Firefox installer. Note: In most cases, you can install a newer version of Firefox over an existing version.

If you install Firefox on a multi-user system where access privileges are restricted, you must run Firefox as a user with access to that location upon installation so that all initial startup files are generated. System requirements for Firefox 3.6 are listed here. If your computer doesn't meet those requirements, you can download a previous Firefox version from the CDN server. System requirements for the current version of Firefox are listed here. Download the Firefox installer from or go to this page to choose the full installer for your language and OS.

Download firefox 3.6 for windows