PERSISTENT FIREFOX POP-UP

Recently, whenever I open saved (or sometimes new web) documents in Firefox, a very large pop-up on the lower right side of my screen appears with a cartoon of a pencil scanning a document. Below the cartoon it states "Now you can highlight your PDFs! With our new highlighter tool, you can read and edit PDFs in one place." The pop-up blocks half the view of the document and cannot be minimized or closed. It also prevents accessing (it hides) the "print" option if I want to print/save the page. I have WINDOWS 11 and have looked online for solutions to this problem which always produces the same suggestion: "go to 'settings', click 'privacy & security', check 'block pop-ups', click 'OK'", all of which I've done. I now limit my use of Firefox to streaming videos via amazon, and use Chrome or Edge for anything else I need to do, but I assume Firefox would not be happy to know of the aggravation their browser is causing, forcing users to go elsewhere (or am I the only person having this problem). Thank you in advance for any new suggestions.