Can you please check the development tools of your browser for any errors, specifically in a view called "Console"? When the number of results are displayed it means that the search itself has completed properly, only an issue in processing the result for display may have caused an error before a change to the UI was made.
That sounds like an issue reported in PDF.js. Can you include a bit more detail (such as any stack trace you can retrieve from the console, your Firefox, Alfresco and Aikau versions)? I am curious - is the error the same in Chrome and IE? It should not because anything with "moz-" is typically Firefox specific...