What folders I must take into account in order to don't loose anything customized from Firefox 11.0 on Windows XP (i.e. Personas, Plugins, preferences).
Is something usually missable when you bring them to Windows 7?
You could just set-up Firefox Sync - This will sync your bookmarks, plug-ins, personas etc - sounds like you are not doing an upgrade so you can set this up and then sync from the Windows 7 installation and check it works ok before you ditch your old setup.
Personally, I prefer MozBackup. It is able to backup anything you want and restore your backups. It works for:Mozilla Firefox, Mozilla Thunderbird, Mozilla Suite and Netscape profiles.