The Acrobat X plug-in for displaying PDFs in FireFox/Chrome does NOT support fast web view. This means that PDFs don't display until after the entire file has been download.
For example, try viewing this document in either IE(Windows) or Safari(Mac):
Notice how it opens very quickly, and then you can scroll around and view all the content as the PDF content strreams in? In Safari you can see the download progress like you used to be able to see in all versions of the acrobat plug-in. Note also that in the IE version, although fast web view is working, you cannot see the download progress like you used to be able to... Thats an issue, but not nearly as bad as the real issue...
Now try clicking the same link in either FireFox or Chrome (note that in Chrome you need to be sure that Chrome's PDF viewer is turned off and the acrobat plug-in is enabled). Note that the PDF is not displayed until the entire file is downloaded.
This feature is VITAL for viewing large documents online. I'm astonished that this has not been reported by other users.
Adobe -- PLEASE be sure that this gets fixed in the next update to Reader X!! I was shocked to see that it was not fixed in 10.1.