Clear the cache and the cookies from sites that cause problems.

• "Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites causing problems: Tools > Options > Privacy > Cookies: "Show Cookies"

Be sure to use the Firefox 4 beta version with its own profile and do not share the profile with other Firefox versions.
Create a new profile exclusively for the 4.0 beta version and create a desktop shortcut with -P "profile" appended to the target to launch that profile.

• http://kb.mozillazine.org/Testing_pre-release_versions

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox

thnx for your reply. i tried what you suggested but im sorry to tell you that it didnt work. However i still thank you for replying.

You're welcome.

Looks like a problem with the Firefox 4. beta 8 version. A test with the Live HTTP Headers extension shows that the user name and password aren't send to the server as part of the POST data in the login form. It works correctly in the current Minefield nightly build, so I'm not sure what kind of problem this is (e.g. a fixed bug that hasn't landed in beta 8). For now you are out of luck with that beta version.

ftp://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-mozilla-central/