Version 1.8 fixes the issues caused by Foursquare's site redesign. Until Firefox approves this add-on's update you can find the latest version here: https://addons.mozilla.org/en-US/firefox/addon/foursquare-checkin/versions/?page=1#version-1.8
After installing the add-on, navigate to any Foursquare venue page and you will see a new "check in" box on the right side of the page. Use the features in this box to check in to the location whose page you are visiting. You have all the options of a regular check in including making the check in private, sending an announcement to Twitter and/or Facebook, and including a shout.
Checking in with this tool counts toward earning points and badges with Foursquare, so feel free to check in as much as you'd like.
This add-on uses my4squarealibi's service for processing checkins.