It seems that the PDF viewer incorrectly takes into account the font settings in Firefox Options, so that if the user chooses to customise the fonts used for viewing web page content, those fonts are also used for displaying PDF documents. Steps to reproduce: 1. Start Firefox using a new profile 2. In Options, go to Content > Fonts & colours > Advanced and untick the "Allow pages to choose their own fonts" checkbox 3. Change the 'Serif' and 'Sans-serif' fonts to something non-standard, eg. Comic Sans MS 4. Open http://www.raib.gov.uk/cms_resources.cfm?file=/130311_B012013_Four_Lane_Ends.pdf Expected results: The PDF should be displayed using the correct font(s) for that document. The same as it would be when viewed in an external viewer. Actual results: The PDF is displayed using whatever weird and wacky font I've chosen to view web pages in. In addition, when any user-selected font is used, certain characters in PDF documents are not displayed correctly, and are replaced by some Chinese or other random character. This seems to happen regardless of which particular font is selected. In the linked PDF for example, all 'fi' and 'fl' characters are displayed incorrectly when the "Allow pages to choose their own fonts" checkbox is unticked.
This bug should actually be a dup of bug 738627, which is unfortunately resolved as WONTFIX.