OK, a week or so ago I reinstalled my operating system, zapping all of my Firefox configuration data. Once I did this, I tried installing Firefox 3.5 again. This time, everything worked without significant problem; I am able to use Firefox 3.5 without significant freeze-ups while loading background tabs.
For other people who are seeing these freeze-ups, try resetting your Firefox profile to see if the problem goes away.
I have released a minor update to Deadwood today.