If more than one person is sharing your version of Firefox you can set up individual profiles to keep your user information separate from the others.
In the search bar type run
Then in the Run box type firefox -p
This starts up Firefox's Create Profile Wizard. Just click on Next.
Here you can put in the name you want on the profile or leave it at it's default or choose another folder to use. Then click on Finish.
Now your at the Choose User Profile box.
You could then just click on Start Firefox to go directly into the browser from there. Or you could Create another, rename one or delete a profile.
If you want your profile manager to open automatically with Firefox, uncheck Use the selected profile without asking at startup. Or leave it checked and Firefox will automatically open up to that new profile.