The best way to measure startup performance is via the about:startup addon.
After installing the addon, go to about:startup to see the measurements. The meaning of gathered numbers is described in the above link.
There are 2 kinds of browser startup to consider: warm and cold.
- Warm startup is when one runs the browser soon after closing it. Typically the operating system has everything cached in RAM during warm startup. To measure warm startup one should close the browser and immediately start it again.
- Cold startup is when the operating systems must load everything from disk. The best way to test cold startup is to reboot your computer in between restarting the browser.