Mozilla tweaked the UI of built-in PDF Viewer in nightly build of Firefox 15 and it certainly looks  better and as a part of Firefox. This Firefox PDF Viewer now loading PDF document much faster than before. If you don’t know about PDF.js aka Built-in PDF Viewer of Firefox: much like to Chrome’s PDF Viewer Mozilla too to offer PDF Viewer for their users, but this is built-on web technologies like CSS3, HTML and  JavaScript. Mozilla Open Sourced their PDF Viewer and it can render most of PDFs right now.

You can also drag and drop PDF file stored on your Computer onto Firefox to open. You can use “Toggle Sidebar” button to view sidebar on left which shows thumbnails of the page. You can zoom in/out the page in PDF by clicking on  ‘_’ ‘+’ controls on the toolbar.You can download the PDF file to your Computer by clicking on down arrow button located on top of right side of the page.

Firefox PDF Viewer with black user interface

Users using Firefox 13 stable version can test this feature  by downloading  this add-on. If  a PDF file can’t be displayed properly then it displays the message on top  that  “This PDF document might not displayed correctly “ and asks user to open with different Viewer much like Chrome says to open with Adobe Reader when it encounters a PDF that it can’t display some parts of the file.