Firefox 15 prevents most memory leaks caused by add-ons, including Firebug. For many users users with add-on installed this will significantly reduce Firefoxs memory consumption, without requiring upgrades to those. Firefox 15 is likely to be faster (sometimes drastically so) and less likely to crash, especially if they have multiple add-ins installed and/or keep Firefox running for a long time between restarts. Leaks in Firefox are not the cause of the problem they are Mozilla’s fault, but the sins of the addons.”]
Source: https://blog.mozilla.org/nnethercote/2012/07/19/firefox-15-plugs-the-add-on-leaks/