They're randomized to make it harder for an attacking program to find the location of it in a worst-case scenario when a Javascript or similar bug gives it read access to the local machine. The script in this scenario would be able to read files it knew the path of but not browse the drive properly. Without randomization, it would be a safe bet to try things like %USERPROFILE%\AppData\Mozilla\Firefox\Profile\Cookies and be able to pull your cookies for examination. If you can't get the USERPROFILE environment variable, the script could also do C:\Users\Admin\... and replace Admin with common names as well like John, Sue, etc. It also helps make it harder for malware to find. Not much harder, mind you, but it does require specific code to look for the Firefox profile as opposed to a single check.