If your web browser (this happened for very recent versions of Firefox and Opera for me) hangs within seconds after launching on Fedora Core 6, check in the System Monitor to see if a process called acroread is consuming virtually all the CPU cycles. Killing the process isn’t sufficient. A new acroread process will be started every time you launch the browser, presumably due to a browser plug-in. There is a solution, though:
# rpm -e acroread
Fortunately, there are plenty of other options for viewing PDFs with Linux.