Feature changes to Firefox 15 included new support for SPDY v3, the Google-designed protocol that promises faster and more secure page loading, and the final installment of the browser's silent update service. Firefox 15 applies regularly-scheduled and emergency updates entirely in the background so that the user no longer sees an update installation progress bar.

Called "background updating" by Mozilla, the process is invisible to users because the update is automatically applied, then staged in a different directory or folder than the current copy of the browser. The next time Firefox is launched, the staged directory swaps places with the active directory.

Mozilla has worked on silent updating, and chased Chrome's similar feature, for over two years.

The addition Mozilla touted, however, was a continuation of more than a year's work on reducing the browser's memory footprint, particularly in plugging "leaks" created when code doesn't properly release memory after a chore is completed. The leaked memory is never returned to the available pool, reducing what's available for other applications, or even for Firefox at a later point. Eventually, performance suffers.

Complaints about Firefox's memory usage have historically centered on the browser's habit of not releasing memory when tabs are closed.