Web browsing (Firefox 1.5.0.6) in Mac OS X (10.4.7) has gotten quite
slow.  Every time I click on a link, the disk spins up and I get the
spinning beachball, and the new page takes a few extra seconds to come
up.  This happened once before, but the problem went away after a few
days.  Any idea what is causing it and how to fix it?

Have you tried resetting firefox ? And/or emptying the cache? 

-> from the firefox menu, click Firefox (upper left), preferences >
privacy. From there, clear the selected data of your choice.

If this does not fix it, you can try dragging the current firefox
program to the trash, re-downloading it, and launching it.

If that doesn't work, open > your hard drive (mine is named Macintosh
HD) > Appplications > Utilities > Disk Utility. Click your drive, and
on the bottom right, verify & repair disk permissions, then restart
the computer, and relaunch Firefox.

Hope this helps

warm regards,
-copperscooper-ga

This did the trick.  Thanks.