From MozillaZine Knowledge Base

If some web pages look wrong or give an error message, you may be seeing a cached version of the page. You can bypass the cache by holding down the Shift key as you click the "Reload" button on the toolbar, or you can clear the browser cache as follows:

• Firefox 3: "Tools -> Options -> Advanced -> Network / Offline Storage -> Clear Now"
• Firefox 2: Tools -> Options -> Advanced -> Network / Cache -> Clear Now"
• Mozilla Suite/SeaMonkey: "Edit -> Preferences -> Advanced -> Cache -> Clear Cache"