After Firefox version 19 was released Mozilla decided to implement a somewhat experimental PDF viewer in the browser which means users don’t have to install a plugin in order to view or download PDFs. Unfortunately experimental seems to be the key word because we have come across issues with fonts being unreadable and after doing a bit of research on Google this seems to be a fairly common issue. According to Mozilla’s own support pages (https://support.mozilla.org/en-US/kb/view-pdf-files-firefox-without-downloading-them), 

“the PDF Viewer may have problems displaying fonts, colours or the whole document. To view these files: Click the download button on the right side of the document header in order to open it with the default PDF program on your computer”.

So Mozilla’s solution is to download the PDF to your computer and open in a stand alone PDF viewer. The other way to resolve this issue is to stop using the experimental PDF viewer Firefox has implemented entirely by changing the default behaviour for opening PDFs. Details of how to do this can be found here – https://support.mozilla.org/en-US/kb/disable-built-pdf-viewer-and-use-another-viewer
Seems a little crazy that Firefox would implement something in their browser that doesn’t always work. Perhaps if there were some sort of warning message to inform people that there could potentially be an issue when viewing a PDF using their built in viewer it would save people from thinking there is something wrong with the document itself. 
For those who are a little more technical, you could be tempted to try a different web browser and if the PDF opens fine then Mozilla could run the risk of putting off potential users from using Firefox in the future.