From MozillaZine Knowledge Base
Mozilla Firefoxâ€™s Places system keeps metadata about browsing sessions in history. This allows the history sidebar to group pages by browsing session as well as by last visit, which can be a useful distinction when trying to find a previously-visited page.
For UI parity with Firefox 2.0, this preference disabled browser session grouping for page history in the history sidebar.
Possible values and their effects
When â€œView → By Last Visitedâ€ is selected in the history sidebar, group the visited pages by browsing session and delineate them with horizontal rules.
Do not group pages in the history sidebar; display them as they appear in Firefox 2.0. (Default)
First checked in
2006-12-08 by Seth Spitzer (patch by Dan Mills)
Has an effect in