Try a clean reinstall of Firefox:
1. Backup your Firefox bookmarks via Bookmarks/Organize Bookmarks/File/Export.
2. Completely remove Firefox via the Add/Remove Programs dialog under Control Panels.
3. Go to the
Firefox site and install a fresh version.
4. If needed, restore your bookmarks with Bookmarks/Organize Bookmarks/File/Import. (the uninstall may remove your bookmarks as well as everything else)
5. Restart your PC. Start up Firefox and see if it connects.
6. If Firefox isn't connecting, follow the procedure linked in my previous message to set your default browser manually to Firefox. If that still doesn't work, manually set the browser to Opera (or do so anyway if you prefer Opera).
When you start up Firefox, what do you see? A blank Firefox window, a spinning icon indicating its trying to connect, or an error message? Knowing this may help us debug the problem.
Vasily