cache can't be cleared for a specific site in Firefox

Solution 1:

What makes you believe that your cache is not being cleared?

the site shows no menu in my Firefox but OK in other browsers

Maybe this is due to a bug in the website causing it to fail loading the menu on Firefox. In which case its no a problem you can fix at your end (except for switching to a browser that works), you need to inform the webmaster of the website to fix it.

Solution 2:

Assuming Windows:

c:\Users\YOURUSERNAME\AppData\Local\Mozilla\Firefox\Profiles\SOMERANDOMSTUFF\Cache

Just trash the contents of that directory while Firefox isn't running.