Firefox 15 prevents most memory leaks caused by add-ons, including Firebug. For many users with add-ons installed this will significantly reduce Firefox’s memory consumption, without requiring upgrades to those add-ons.

For those users, Firefox 15 is likely to be faster (sometimes drastically so) and less likely to crash, especially if they have multiple add-ons installed and/or keep Firefox running for a long time between restarts.
https://blog.mozilla.org/nnethercote...-add-on-leaks/