Firefox

C:\Documents and Settings\<Profile name>\Application Data\Mozilla\Firefox\Profiles\<gibberish>.default\bookmarks.html

Internet Explorer

C:\Documents and Settings\<Profile name>\Favorites

Note that unlike Firefox/Mozilla bookmarks, each favorite in Internet Explorer is a separate file. Be sure to back up the entire contents of the Favorites folder!

Mozilla

C:\Documents and Settings\<Profile name>\Application Data\Mozilla\Profiles\default\<gibberish>.slt\bookmarks.html