create a panel and insert to chrom document. then open it with this code: https://developer.mozilla.org/en-US/docs/XUL/Method/openPopup
set anchor to gBrowser.selectedTab.linkedBrowser
set panel width and height to same as the linked browser so: panel.height = gBrowser.selectedTab.linkedBrowser.getBoundingClientRect().height do same for width
https://developer.mozilla.org/en-US/docs/XUL/panel
if u need more help let me know ill write the code, its real simple