To clear cookies, cache and history in Firefox 3.6:
1. Open Firefox.
2. Click the Tools menu and select Clear Recent History.
3. Click the dropdown menu Time range to clear: and select the range of history you'd like to delete and indicate the Details you'd like to delete by checking the appropriate check boxes.
4. Click Clear Now.
Poof! Your cookies, cache and history have disappeared in Firefox 3.6.