It has been years since I last had to do this but find the profile directory for Firefox and copy it to wherever you want to back it up. This link should give you all the info you need about where the directory is and how to restore all the settings.
What I did a long time ago was to backup the profile directory and then placed it on a different drive/partition from my OS drive/partition. After I reinstalled the OS I would install Firefox and the first time I started it up I would use the -profilemanager switch. This allowed me to point Firefox at the directory where I had my profile stored and everything was back up and running. You can use the exact same process with Thunderbird if you use it for an email client.
i would copy the entire mozilla folder in the appdata. you can get to this location by typing %AppData% in the windows search bar. that folder should hold all your bookmarks, passwords, extensions, etc. you can overwrite another computer's mozilla folder in that computer's appdata location and get a "clone" of the original firefox. at least that's how i recently did it.
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