Looking at the html of the Pathfinder Sheet, and the mozilla docs: <a href="https://developer.mozilla.org/en-US/docs/Web/Security/Securing_your_site/Turning_off_form_autocompletion" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/Security/Securing_your_site/Turning_off_form_autocompletion</a> I can see that the roll20 Form element is marked as autocomplete="off" , and not the individual input elements. I'm going to guess that some recent Firefox update changed how this is handled in a subtle way, causing these weird performance impacts. I followed the suggestion to globally disable form filling, and performance returned. My off the wall suggestion is to add autocomplete="off" to every input field, and hope Firefox takes the clue.