From MozillaZine Knowledge Base
A roaming profile is a collection of user settings which can be seamlessly shared between different machines or applications, or different instances of the same application. This can be useful for users who wish to work in a familiar environment regardless of the specific machine or application they make use of, and who have personal data to which they always wish to have access.
SeaMonkey releasess since version 1.0 include roaming profile support as a program feature, although buggy at present. See Roaming profiles - SeaMonkey for details. Other Mozilla applications do not currently support roaming profiles as a program feature. As an alternative, you can approximate roaming profiles or share a fixed profile.
How can I share a fixed profile, such as on dual-boot machines?
This is very easy. Supposing your profile exists for one particular instance of a Mozilla product, you need to make use of the Profile Manager for the other instance of this application in order to inform it of where your existing profile is stored. Simply choose to create a new profile from within the Profile Manager; when prompted for the installation folder browse to and select your existing profile folder. Rather than creating a brand new profile, your application will instead recognize the existing profile and allow you to use it.
How can I best approximate roaming?
Due to the nature in which user profiles are created and stored, it is at least possible to transfer one between different instances of your Mozilla product by making some small manual configuration changes. (This mechanism also facilitates easy backups.)
Structurally, a profile is a self-contained unit stored in a physical folder in your file system. From Firefox 1.5 onwards it is possible to move/copy an entire profile folder from one location to another by simply copying it to the new location and using the Profile Manager to register it using the method, above, for sharing a profile. This will work for Thunderbird profiles also, if the profile was originally created in Thunderbird 1.5 or higher.
Thunderbird 1.5 now uses relative paths to the mail folder files, so a Thunderbird profile that was created in Thunderbird 1.5 can be moved with no problems. When Thunderbird 1.5 uses a profile created in a Thunderbird 1.0.x version for the first time it generates and stores the relative paths to the mail folder files, but it does not delete the old absolute paths; and it will use the absolute paths before the relative paths if they are present. If the settings contain absolute paths, they must be deleted before the profile can be moved.
Thus the nearest one can currently get to roaming is to copy your profile onto a removable disk, modify the locations recorded in the files to reflect the location of the profile folder on the disk (if needed), and use the Profile Manager for the same Mozilla application on different machines to create a profile which points to your portable profile folder (see Moving your profile folder for an illustration). Of course, the drive letter which gets assigned to your disk on Windows machines may vary, in which case you will need to modify the files each time to reflect this.
Full installation onto removable disks
It is also possible to go one step further and install Mozilla products (and their associated profiles) directly onto removable disks, making your internet experience truly portable.
Related bug reports