Lately I've noticed that for certain PDFs (those served in a web page) in Firefox 4.0b10, that the page is simply blank. There's nothing but the normal Firefox background. Acrord32.exe never runs. If I kill the plugin-container.exe process it simply says the Adobe plugin crashed and asks if I want to reload. If do I get the same blank page again.
I have to go and turn off protected mode, kill the plugin-container.exe process again and then the PDF will load. This problem seems to occur with PDF files that load solely in the plugin-container.exe and don't launch acrord32.exe. They also have the old Reader 9 style toolbar at the top of the page instead of the floating toolbar in the middle of the page.