Several months ago, there was a bug where Firefox 21 stopped updating the Jump list. When I researched it around that time, I reached the following conclusions:

(1) The "Frequent" section of the Jump List matches the top items listed by the "Most visited" bookmark ("smart folder").

(2) Firefox maintains a binary data file in this folder (because it's a binary file, it can't easily be edited):

%APPDATA%\Microsoft\Windows\Recent\CustomDestinations 

(My file is named 969252ce11249fdd.customDestinations-ms but yours might have a different name... not sure how these names are chosen)

Accordingly, I'm not optimistic about gaining control over the selection of items in the Frequent section of the Jump list.

You can add bookmarks to your Firefox Jump List, but in order to control the names (by default, the URL is displayed), you need to use a two step process:

(1) Drag bookmark to the desktop (or another Windows Explorer folder), and rename as desired.

(2) Drag and drop onto the pinned Firefox icon, then right-click to confirm it was added to the Pinned section.

After this, you can delete the desktop shortcut.

If you want to omit individual sites from the Frequent section, you can right-click and choose "Remove from this list".

If you want to omit the Frequent section (or other sections) completely, or change their lengths, you can adjust the Jump list preferences as follows.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box that appears above the list, type or paste task and pause while the list is filtered

Double-click to flip between true and false:

browser.taskbar.lists.enabled default: true
browser.taskbar.lists.frequent.enabled default: true
browser.taskbar.lists.recent.enabled default: false
browser.taskbar.lists.tasks.enabled default: true

Double-click to edit the value:

browser.taskbar.lists.maxListItemCount default: 7 (max:10?)

Thanks...worked beautifully! I bookmarked the page, drug that to the desktop then renamed it. Then drug from the desktop right onto the FF icon pinned to the task bar.

Thanks..