I just built a new machine and installing Acrobat was last step in process. Am running Thinkpad X210 with 10.10 64-bit. I've always been able to reliably open PDFs inline from the web; however, when I launch a PDF, the main Firefox panel goes either white, completely black, or white with black diaganol streaks.
Perhaps this is video related... Here's what I have chipset wise
VGA compatible controller: Intel Corporation Core Processor Integrated Graphics Controller (rev 02)
I'm certainly not wed to Acrobat, but I do want to have Firefox with inline PDFs from web.
You could try to embed Evince, the default PDF reader, into Firefox. To do this install the mozplugger package.
You can then create a file in the '.mozilla' folder in your home folder called 'mozpluggerrc' and edit the contents to add the following:
Mozplugger Documentation