To downgrade to Firefox 3.6 first uninstall Firefox 4, but do not select the option to "Remove my Firefox personal data". If you select that option it will delete your bookmarks, passwords and other user data.

You can then install the latest version of Firefox 3.6 available from http://www.mozilla.com/en-US/firefox/all-older.html - it will automatically use your current bookmarks, passwords etc.

To avoid possible problems with downgrading, I recommend going to your profile folder and deleting the following files if they exist - extensions.cache, extensions.rdf, extensions.ini, extensions.sqlite and localstore.rdf. Deleting these files will force Firefox to rebuild the list of installed extensions, checking their compatibility, and reset toolbar customizations.

For details of how to find your profile folder see https://support.mozilla.com/kb/Profiles

If you just need it for the one site, an alternative is to keep Firefox 4 but install the portable version of Firefox 3.6 on your hard drive. You can then use this version to access that site. You can get the portable version from http://portableapps.com/apps/internet/firefox_portable - use the legacy version links near the bottom of that page.

Thank You, but it did not work; I think that the site was built to work only with the old versions of IE.

Try using IE Tab2.
https://addons.mozilla.org/en-US/firefox/addon/92382

Sorry, still does not work. See bellow: Firefox doesn't know how to open this address, because the protocol (vbscript) isn't associated with any program.