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.
Michael Lane's Technology Blog 
doesn’t work. Nice feature, but entries are outdated. Can’t even get rid of it.
Search on “How Clear/Disable Complete Recent List in Taskbar Jump List of windows 7” or “10”
https://www.groovypost.com/howto/clear-windows-10-jump-lists/
Yes, I found the same thing. It just didnt work. Removing the recent list from the task bar menu is a pain because none of the suggested ideas work.
may require win10 pro for some additional customization of taskbars.
i upgraded to win10 pro and doing the steps to clear taskbar freq lists works like a charm now.
worth getting win10 pro – just for this? (likely not)