Here is a quick way to see performance as well as overall stability statistics of Mozilla Firefox :
In the address bar, type :
Or go to Help > Firefox Health Report.
This will display a detailed graph that shows various parameters like the time taken by Firefox to load (on a day by day basis), the date when it last crashed, the time duration for which it has been running, the version number and so on.
Looking at this report is also a quick way to see if performance of Firefox has slowed down considerably or is fairly the same and can be useful in troubleshooting (say if Firefox starts up slowly time after installing some add-ons, it can be quickly viewed here and fixed accordingly).