There is another way to manually export the bookmarks from IE7 / IE6 and then import the bookmarks in firefox. For this, in IE7/IE 6, click add to favorites. Now click import and export. Here in this wizard hit 'next' . Now click favorites and hit next. Now select what favorite folders you want to export. If you want to export all bookmarks, click top level favorites folder and hit next. Now it'll create a file called 'bookmarks.htm' which'll have your favorites. This by default will be created in My Documents folder (choose a different path if you don't want it to be exported in My Documents) and hit next. Finally click finish.
Now that we got the IE7 favorites exported to bookmarks.htm, all we got to do is import them into firefox. So in firefox, click bookmarks -> organize bookmarks . In the bookmark manager window , click file->import . Now here select 'from file' and hit next. Now browse to where the exported 'bookmarks.htm' is located(the file we exported from IE7 / IE6).This will import the favorites entries in bookmarks.htm into firefox.