Lightweight Mavericks installation
Go to Activity Monitor's Memory tab and sort the list of processes by Memory. You'll have a complete list of your biggest RAM hogs, complete with the ability to select and quit them right from Activity Monitor.

Also, Memory Diag is a great app for freeing up RAM. Just click on its menu bar icon, click the big Optimize button (provided you're not at critical RAM usage), and watch the RAM free up!

Also, I highly recommend Safari instead of Firefox. Safari shows you the individual websites in Activity Monitor, so you can just quit the most RAM-hungry sites.
(EDIT: It shows you the actual URLs in Yosemite. In Mavericks, you'll see several Safari Web Content processes, but you can hover over them to see the URLs.)