From MozillaZine Knowledge Base

This issue was reported in Firefox 1.5 and may no longer apply to Firefox 2 or above.

• A corrupt localstore.rdf file in your Firefox profile folder can cause the Add Bookmark dialog to fail. [1] To fix it, see this article.
• The Bookmark Duplicate Detector extension may also cause the Add Bookmark dialog to fail [2]. If updating the extension doesn't help you should uninstall it.

In Firefox 3, if bookmarking a page has no effect see Locked or damaged places.sqlite.