In order to switch to another persona that you had previously installed, do the following:
- Go to Tools and click Add-ons (or click the Firefox button at the top left and click Add-ons).
- Switch to the Appearance tab. You should find your previously used personas in the list.
- Find the one you want to use and click the Enable button.
To uninstall the other ones you can click the Remove button on them and they'll be uninstalled instantly.