Why use different profiles? Different profiles are useful if more than one person is using Firefox browser on a computer. Each one can configure Firefox as per their needs
It is also useful if you require multiple instances of log-in to services like Gmail, Hotmail and other accounts.
To synchronize between two profiles you required installing Dropbox and creating a new account with them. It is free for up to 2GB of online space.
More information and download is available from Mozilla add-on site.