Not that I am aware of unless you site provides a Descending/Ascending order option there is no way to do it in FF itself.

How is the list displayed? I have to assume it's not all one long page, but if it is, you might be able to inject a script into the page to make it sortable. Most commonly, individual items would either be:

• <tr> elements (table rows)
• <li> elements (list items in an ordered or unordered list)

The code would vary based on the fine details of the layout.

Any sorting done this way most likely would be temporary until you reload the page and probably not possible to save back to the site. If you need to update the data permanently, some kind of export/import probably would make more sense, if the site supports that.