If you still want to downgrade to Firefox 9, follow these steps:
1> Firefox 9 is not available directly on the website of Mozilla (the company behind this software). But, you can get installer of Firefox 9 from Mozilla’s FTP servers.
2> Run the installer file, and it will downgrade the Firefox 10 to Firefox 9.
3> If you want a “clean” installation:
a> Take proper backup of your bookmarks, saved passwords etc. You can use Mozbackup for this.
b> Uninstall the Firefox completely. Select to delete browsing data when asked during uninstallation. To uninstall Firefox, visit “Control Panel” in Windows.
c> Install the Firefox 9 now.
d> After installation, run Mozbackup again to restore the backup data.