A suggested method to correct it is to use external PDF viewer as suggested at Mozilla.
Web searches yield lots of discussions and the discussions are mostly how to find a way to display it correctly just like the above. Those discussions are on the "consumer" side.
However, if you are the on the side of producing and posting PDF documents online, you are probably more interested from beginning with in producing PDF documents that can be displayed correctly by the Firefox Build-in PDF Viewer.
As I investigate, this is not a trivial task. Generally, when the PDF Viewer does not display the document correctly, it is a result that the document contains certain feature which the viewer does not support. To generate a compatible PDF document to the viewer is not to use any unsupported feature that the viewer does not support. However, I failed to locate any document showing which feature is supported and which is not, including the project site at GitHub.
One method that worked for me at least in a few occasions is to use Firefox's Element Inspector.
When a PDF document is not displayed correctly, locate the element that is not displayed correctly or a nearby element to the element that is invisible as a result of not being displayed correctly, right-click the mouse and select "Inspect Element". Examine the element and elements nearby in the Inspector. If you examine them carefully, you may identify something suspicious. For instance, in FireFox 26.0, a PDF file saved in Microsoft PowerPoint is not displayed correctly. Examining it in the Element Inspector, I found out that the location of the problem and suspected that the font "Verdana" somehow could not be displayed correctly, as shown below.
By replacing font "Verdana" by a recognizable font, the problem went away.