Firefox certainly isn't as smooth as you would like it to be, at least in some situations. But sometimes, these problems are hard to reproduce for developers because they only appear under some circumstances.
The Gecko profiler empowers users and allows them to either give important profiling data to developers, or track the root cause of their performance problems themselves.
Other times, Firefox is not entirely to blame, and scripts on some web sites are just too slow. The Gecko profiler also allows to profile content performance.
Mike Hommey introduces the Gecko profiler, and shows how to use it in different scenarios, and how to use it to get useful data from your desktop browser or even Firefox for Android.
Recorded live in the Mozilla Devroom on Day 1 of FOSDEM '13 in Brussels.