^ You might try the "PDF Download" extension. When you click on a pdf link, it will pop up asking if you want to download it as a file, view it in an online pdf-to-html converter, or simply open it up in the browser.
Personally, FF3 has been running quite well for me. The only slowdowns I get are with AJAX-heavy websites, but I rarely get any complete freezes. I'm actually running quite a number of extensions too:
Adblock Plus
All-in-One Sidebar
CustomizeGoogle
DownThemAll
DragDropUpload
FireFTP
Gmail Manager
Google Preview
Greasemonkey
Linkification
Menu Editor
Minimap
Mobile Barcoder
Organize Status Bar
PDF Download
Personal Menu
Read it Later
RefControl
ReloadEvery
Screengrab
Stylish
Tab Scope
I was previously using NoScript out of security concerns but frankly NoScript's algorithms simply are not optimized. It actually caused FF to slow down more in trying to filter through all the blocked pages rather than to simply let them all load as normal.
I agree FF3 isn't quite as major an upgrade to FF2 (although in my experience FF3 manages its memory usage better), but I think this will change with FF3.1 when TraceMonkey (the new javascript engine) is released. With TraceMonkey, AJAX-heavy pages will perform significantly faster.