Firefox 15 doesn't look any different, but Mozilla claims users will see "drastic improvements in performance" because the browser stops third-party add-ons from leaking memory like the proverbial sieve.

In June 2011, an effort to plug those leaks. With Firefox's own problems addressed -- in a today, Asa Dotzler, director of Firefox, said Mozilla has "fixed the larger Firefox issues" -- the company turned attention to third-party add-ons.

"It was time to tackle the next big source of memory leak: poorly written add-ons," said Dotzler.

Although engineers worked with some add-on developers one-on-one, Mozilla could not scale that effort. Instead, it crafted a leak-prevention mechanism that blocked the most common kind of add-on memory mistake, in which the extra copies of a website aren't released after a tab has closed.

"These pages pile up, and can eat massive amounts of memory for no user benefit. They leak," Dotzler said.