Is this a problem with the Pinned section of the Jump List for the Firefox icon? Assuming so, a few questions:

• Did anything significant happen before the data was lost, such as using the Reset feature or uninstalling/reinstalling Firefox?

• Are other sections of the Jump List (such as Frequent and Tasks) operating normally?

• Are you able to pin a new site to the Jump List?

Windows uses a binary file format for Jump List data, meaning that viewing it in a text editor reveals a lot of gibberish in addition to scattered text (if you can even determine which file is relevant). There are utilities to extract this data. I haven't tried any myself, but this one came up in a search: http://www.nirsoft.net/utils/jump_lists_view.html

Or, if the problem is with Firefox's bookmarks toolbar that typically appears below the main navigation toolbar, see this article: Bookmarks Toolbar - Display your favorite websites at the top of the Firefox window.