It is always recommended to uninstall and completely remove the folders of the older versions first before install the newer ones. I supposed you skipped that part.
Anyway, I was playing around with some old extensions and it completely crashed my Firefox and refuse to restart again. What did I do? I uninstalled Firefox, and deleted the extension folder in my profile. Reinstall the latest version and it was back to normal. I could have swore that it runs much faster now. Some older extensions like Ad-Block are known to cause some compatiblity problem with some sites. I am not sure if the latest version manage to solve that but I am in no hurry to give it a try.
