Are you talking about bookmarks ? That is what Firefox calls them. If you are talking about Favourites that is something else on Windows or Internet Explorer and Firefox may be offering to import them.

When you talk about your account I wonder if you are using Firefox Sync that will by default offer to sync your bookmarks between devices and accounts.

• How do I set up Sync on my computer?
• How do I choose what information to sync on Firefox?

If you have a new Windows account for some new user that will be likely to include Firefox, but it should have a new profile that does not use your settings and profile, and will not have your bookmarks, unless you decide to use Firefox Sync to sync your bookmarks.

I note by the way you appear to be using the Firefox Beta channel, that is a pre release channel and will update more frequently than the normal Release channel. So you are on Firefox 48.0b whereas most people will be on Fx47.

It's not a problem with windows. I use Firefox 48.0b9. I use the Firefox Sync for Bookmarks on my 2 computers and my phone. My unsolved problem is: I sold one of my computer and when I disconnect my Firefox account of this machine, the new user can see ever my Bookmarks. How I may disconnect my account and connect the new user on this machine. When I so make, the synchronization merges the bookmarks of the new user with my bookmarks? THX

Take the old computer. Shut down sync on it. After, remove all profiles from that system.

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles