To find out the version and build number of PDF.js (built into your installed version of Mozilla Firefox), you should visit a specific URL (if it doesn't work, just try to copy-and-paste it):
and look patiently at the beginning of that page.
E. g., for Mozilla Firefox
38.0.5 we can get the following:
...
PDFJS.version = '1.0.1149';
PDFJS.build = 'bc7a110';
...
(To view more of its internals, just visit
resource://pdf.js/. Note: if you browse through that, the address will expand to a specific path pointing to some file inside some library-archive of Firefox, e. g.,
jar:file:///path/to/firefox/dir/omni.ja!/chrome/pdfjs/content/build/pdf.js).
Links:
