Mozilla profile is a special folder where your user data is stored. This includes Firefox bookmarks, Firefox settings, downloads and browsing history, passwords, site-specific preferences, search engines, personal dictionary, autocomplete history, cookies, etc. So if something happens to the browser, you can go to the profile folder and restore Firefox preferences in a couple of clicks.
Similarly Mozilla’s mail client – Thunderbird, keeps your local mail, account settings and the toolbar in a separate folder.
Profile folders allow you to customize the web-browser and mail client on any computer in no time, so if you keep them online, you can download it anywhere anytime and apply the same settings on another computer and device. To move profile folder of Firefox/ Thunderbird to a cloud you will need CloudMounter.