See:

• http://kb.mozillazine.org/browser.sessionstore.max_tabs_undo
• http://kb.mozillazine.org/browser.sessionstore.max_windows_undo

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold(user set).
• Preferences can be reset to the default or changed via the right-click context menu.

thanks for helping a novice out! can i also increase number of last visited sites in history menu--i don't see it. it would be really great if i could also easily delete selected items from these lists---is that possible??

    i use these features a lot, mainly because it seems having lots of tabs/windows open at once slows things down--is that right?

You're welcome.

If you mean the list that you see in the History menu than that number can't be increased.

You can create a bookmark to see more Recently Visited sites

• place:queryType=0&sort=4&maxResults=30

See also:

• https://developer.mozilla.org/en/Places_query_URIs