There is nothing that annoys me more in Firefox when a PDF document tries to open and crashes the browser.
PDF Download prevents this problem ever arising by allowing you to control exactly how and when PDF files are opened or downloaded. Instead of opening PDF files "blind" which is currently the case, PDF Download lets you know exactly how big it is with options of whether to convert it, open it, download it or convert to HTML. PDF Download stops the annoying problem of PDF files taking ages to open - especially big ones. With PDF Download, you can see instantly whether its a big file, in which case, it's advisable to download it for reading later as opening a big PDF file is like waiting for Christmas to arrive sometimes.
The PDF Download plugin usually presents you with 3 options unless you've instructed it to do the same thing by default all the time - convert the PDF to HTML, download the PDF or open the PDF in the browser. One bonus with PDF Download, that perhaps separates it from others, is that it's not just limited to Adobe Reader. It also works with Foxit Reader and other PDF readers although to be honest, if you're using Foxit, you won't find much use for PDF Download anyway as it opens PDF files much more smoothly than Adobe Reader.
PDF Download is an excellent PDF tool which will result in fewer Firefox crashes although not everytime as it still seems susceptible to crashes when opening PDF files in the browser.