Debug Builds
Debug builds (--enable-debug) and non-optimized builds (--disable-optimize) are much slower. Any performance metrics gathered by such builds are largely unrelated to what would be found in a release browser.
GC Poisoning
Many Firefox builds have a diagnostic tool that causes crashes to happen sooner and produce much more actionable information, but also slow down regular usage substantially. In particular, "GC poisoning" is used in all debug builds, and in optimized Nightly builds (but not opt Developer Edition or Beta builds). The poisoning can be disabled by setting the environment variable
JSGC_DISABLE_POISONING=1
before starting the browser.
Async Stacks
Another option that is on by default in non-release builds is the preference javascript.options.asyncstack, which provides better debugging information to developers. Set it to false to match a release build. (This may be disabled for many situations in the future. See bug 1280819.
Accelerated Graphics
Especially on Linux, accelerated graphics can sometimes lead to severe performance problems even if things look ok visually. Normally you would want to leave acceleration enabled while profiling, but on Linux you may wish to disable accelerated graphics (Preferences -> Advanced -> General -> Use hardware acceleration when available).
Flash Plugin
If you are profiling real websites, you should disable the Adobe Flash plugin so you are testing Firefox code and not Flash jank problems. In about:addons > Plugins, set Shockwave Flash to "Never Activate".
Profiling tools
Currently the Gecko Profiler has limitations in the UI for inverted call stack top function analysis which is very useful for finding heavy functions that call into a whole bunch of code. Currently such functions may be easy to miss looking at a profile, so feel free to also use your favorite native profiler. It also lacks features such as instruction level profiling which can be helpful in low level profiling, or finding the hot loop inside a large function, etc. Some example tools include Instruments on OSX (part of XCode), RotateRight Zoom on Linux (uses perf underneath), and Intel VTune on Windows or Linux.