hello, the pdf viewer is still somewhat experimental and under very active development - you could try if the same error is occurring in the current development version, which you can download as an addon from http://mozilla.github.io/pdf.js/extensions/firefox/pdf.js.xpi

in case it has the the same rendering problems, you could then file a bug report at https://github.com/mozilla/pdf.js/issues (a github account is necessary for it though) - please also provide a sample document where the error is apparent!

my system already has the pdf update still no good .love the firefox browser and using it for years but that pdf the last updates in the fire fox lets it down ...pdf viewer works great I E, and not firefox

If the built-in viewer can't handle a large number of the PDFs you view, you can disable it an use the Adobe Acrobat plugin instead. See: View PDF files using Firefox’s built-in viewer.