As described in the Profiles article, Thunderbird saves personal information such as messages, passwords and user preferences in a set of files called a "profile", which is stored in a separate location from the Thunderbird program files.
When you install Thunderbird it creates a profile called "default". This profile is used automatically. However, you can create additional profiles, each with their own set of account configurations, messages, etc. (Note, however, that you can access multiple email accounts within a single profile as well.)
The Profile Manager is used to create and delete profiles, and to select the profile to use for a Thunderbird session.
The Profile Manager is not displayed by default. Instead, you must start Thunderbird with a "switch" that makes the Profile Manager appear:
thunderbird.exe -p" and click OK.
On 32-bit Windows
"C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -p
On 64-bit Windows
"C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe" -p
/Applications/Thunderbird.app/Contents/MacOS/thund" and press Return.
If the Profile Manager window does not open, Thunderbird may have been running in the background, even though it was not visible. Close all instances of Thunderbird or restart the computer and then try again.
To create a new profile, click the Create Profile... button. After specifying a name for the profile, you may also specify a custom location for the profile folder (or click FinishDone to use the default location).
To rename a profile, highlight the profile in the Choose User Profile window and select Rename Profile.... Give the profile a new name and click OK.
To delete a profile, highlight the profile in the Choose User Profile window and select Delete Profile.... Click Don't Delete Files if you only want to remove the profile from the list but keep its files. Click Delete Files to permanently delete the profile and all its data.
Choosing this option loads the selected profile and starts Thunderbird without connecting to the Internet. You can view messages saved to your computer and experiment with your profile.
When you have multiple profiles, this option tells Thunderbird what to do at startup:
See the Profiles article for information about locating the profile data files on a file system and making backups.