Firefox 5 is a security update for Firefox 4, and none of the bookmarks features are different between the two versions. Plus, when there are differences between major versions of Firefox, the correct information is automatically selected for the browser version being used when accessing the support pages.

On the right-side of each support page there is a control under the heading Help with: which can be used to select different version of the same support article for different operating systems and different versions of Firefox.

Have you read this support article?
https://support.mozilla.com/en-US/kb/Lost%20Bookmarks

What have you tried from that article?

A possible cause is a problem with the file places.sqlite that stores the bookmarks and the history.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• http://kb.mozillazine.org/Lost_bookmarks