Exactly what I was searching for Rated 5 out of 5 stars

I have used PerformanceTest to check the startup time at each new major release from Firefox 5.0.

Before that, I have used startup.html from http://blog.vlad1.com/2009/07/28/measuring-startup.
PerformanceTest is based on the same principle but computes automatically the mean time of n startups (n is controlled by a script; see Developer's page http://www.teesoft.info/addon-performance-test/?from=-1&to=1.0).

For better measures,
set the Firefox general option 'When Firefox starts' to 'Start with a blank page' (browser.startup.page = 0)
and avoid to have any active Panorama session.

Generally speaking the use must be reserved for advanced users, with notions of scripting & python and who can identify a Firefox profile.