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.