usually whenever a page of a website is bookmarked, a blue star is visible at the corner of the address bar, mentioning "edit this bookmark" when you hover on it. however while browsing thru a google doc, depending on which paragraph the cursor is located, a URI fragment such as "#heading=h.xxxx" being added. it appears that firefox is too strict about such a minor change in the URI, causing firefox to stop detecting the page as bookmarked, and displaying an empty star mentioning "Bookmark this page" instead. this behavior is observed in google docs, but is not exclusive to any specific website.