Mozilla and other browser vendors are diligent about fixing these as they come up. Internet Explorer 6 had notorious internal leak hazards (closures as DOM event handlers), but Microsoft fixed them in IE9 and IE10.
InfoWorld: What problems result from these leaks?
Eich: Developers shouldn't put all objects in a big table or other similar structure such as a linked list.
InfoWorld: Once leaks occur, what can be done to fix them?
Memory tools can also help. A memory profiler can show which objects are taking up the most memory. Then, a garbage-collection tracing tool can tell which program values are causing those objects to be kept.