A Firefox 4.0 version is currently in the queue for review by Mozilla. In the mean time you can download it directly via http://foursquarefox.com
foursquarefox is a Mozilla Firefox Add-on uses wifi location to enable users to enjoy the features of foursquare without having to use a mobile phone. If you enjoy checking into places, seeing who else is there and earning the right to be mayor, then this add-on makes is super easy to do.
For more info go to http://foursquarefox.com or follow http://twitter.com/foursquarefox
This add-on uses affiliate revenue to partially fund development. It follows best practices for affiliate linking, only adding a code when none currently exists. For example, if you visit a site with a link that is eligible for affiliate revenue, and already has an affiliate code attached to it, foursquarefox does nothing to that link. However, if a link, that is eligible for affiliate revenue, and does not have an affiliate code, foursquarefox appends the appropriate affiliate code to the link. This way it plays nicely with the rules of affiliate linking.