Firefox has a jump-list of frequent used web pages when you right click the Firefox taskbar icon:
This can be disabled as follows.
Type about:config into the address bar and then press Enter.
Note the warning displayed and then click the I’ll be careful, I promise! button to continue.
In the search bar type frequent and then right click on browser.taskbar.lists.frequent.enabled and then click on Toggle from the drop down menu.
The value will change from true to false.
The change will take effect immediately.