Firefox will load Facebook but usually only after a hesitation of 15 seconds or more so not over convincing. I can't find any other website in IE9 or Firefox that doesn't open instantly. I have 30MB broadband and a clean install. I have no spyware/adware so this is a strange one.
Try checking the security level settings and reset all the security zones to default.
If you configure security settings to be too restrictive, you may prevent Internet Explorer from displaying certain Web sites. To determine whether an issue is caused by overly restrictive security settings, revert to default security settings. To do this, follow these steps:
a. Open Internet Explorer.
b. Click Tools, and then click Internet Options.
c. Click the Security tab.
d. Click reset all zones to default level, and then click OK.
After you do this, test Internet Explorer to verify that it works correctly.
Strange it should happen just for this one site. Normally clearing "cookies" etc. which you have already done usually solves the problem. Something is blocking it - have you checked your Firewall settings to see if Facebook is set to "allow"?