If you have added a lot of personas to Firefox, and some of them are no longer available in the Add-ons window, it is because Firefox only keeps your 8 most recently used personas.
If you have more than 8 personas, you can either bookmark each personas page or keep track of them with a free personas account.
Bookmarks let you save links to websites you've visited. If you want to use a persona that is no longer in the Add-ons window, bookmark the persona, and visit that page when you want to use it.
With a free personas account, you can keep a list of more than 8 favorite personas.