Firefox's expandability through add-ons is often cited as a key feature of the browser, but it is also known to be a source of sluggish performance and slow startup times, particularly when running several add-ons simultaneously. Things have notably improved with the release of Within the next two weeks, Mozilla will add a warning to any add-on that slows Firefox's startup time by more than 25%, and in an upcoming version of the browser, third-party add-ons from standalone software vendors will not be installed unless the user specifically approves the installation (think browser toolbars and the like). This should have a significant impact on Firefox performance and give users back the control they should have over their add-ons, according to Scott.