How can I clear a single site from the cache in Firefox?

1. Select the Firefox Menu button (three horizontal lines on the right).
2. Select Library.
3. Select History.
4. Scroll to the bottom and select Show All History.
5. Search for the site using the Search History field in the upper-right corner.
6. Within the results, right-click the appropriate page and select Forget About This Site.

Note: This will clear saved passwords for the site as well.

If you want to do a force-refresh, which is what I think you want to do, hit CTRL+F5. And of course, Cmd+F5 on a Mac. Works on Chrome too.

this addon will allow you view and delete specific cache entries from memory and disk - https://addons.mozilla.org/en-US/firefox/addon/2489/