While the Firefox web browser does not have that feature build in there is a Greasemonkey script with a similar functionality. Page Scroll is a script for the Greasemonkey extension for the Firefox web browser that will display transparent lines on all websites. It will display the lines on top by default and display the other lines if the user is working with the Page UP / Page Down keys, the mouse wheel or by dragging the scroll bar around.

This is however the only known script that is mimicking Opera’s Scroll Marker feature. Some users might not like the fact that the script is active on all websites by default. A right-click on the Greasemonkey icon and a selection of Manage User Scripts makes it possible to customize the sites list. Change the wildcard symbol to urls to activate the script only on those selected urls. Make sure to use wildcards yourself if the domain is making use of subdomains and directories.

Please note that the script seems to have difficulties on some websites to display the transparent scroll lines correctly. It should work on most sites perfectly however. Google Chrome users might get the script working in their web browser as well if they check the Google Chrome Greasemonkey article.