"... The places file, just like any other file on the system, can become corrupt. This can happen for instance after a crash of the browser or system.Source: Ghacks Technology Newsletter
"The effect of this corruption may vary, but it can happen that you can only access part of the bookmarks, or notice that Firefox does not record any new browsing history entries anymore.
"The Firefox extension Places Maintenance has been designed to provide users with an easy to use way to resolve any bookmarks or history issues in the web browser. ..."
Details Resolve Firefox bookmarks and history issues with Places Maintenance