When you reset Firefox then a new profile is created and some personal data (bookmarks, history, cookies, passwords, form data) is automatically imported and the current profile folder will be moved to the desktop to an "Old Firefox Data" folder. Installed extensions and other customizations (toolbars, prefs) that you have made are lost and need to be redone. It is possible to recover data from the old profile. Check this article to know the way: [[Recovering important data from an old profile]] Above mentioned article may seem overwhelming at first glance, but actually it is very easy to follow and won't take much of your time.

Well said.