Both FoxyProxy versions are free and offer essentially the same features. The only difference is that the FoxyProxy Standard version has a URL Patterns function.
Instead of using proxies on all target domains, the URL Patterns function allows you to specify which websites should be accessed with the proxy by whitelisting or blacklisting the desired domains. This setting uses regular expressions, which is extremely useful when, for example, you do not want to use proxies to access specific web pages on a site or vice versa.