Going by this:
http://www.google.com/support/forum/p/Web+Search/thread?tid=41cc2faa1537c68d&hl=en I found the wdmaud.sys in my c/windows/system32 folder where it didn't belong. I deleted it (destroyed it with a program called Incinerator actually), and lo and behold FF started working properly.
Then I restarted my computer, and tried IE - the same URL substitution problem! I check the system32 folder, and wdmaud.sys is back. I deleted it a second time, and both FF and IE are working properly... at least until I restart my computer.
I couldn't find the sysaudio.sys file referenced in the link above.
Apparently one person posting in the discussion linked above believes he got this through the same old version of Adobe that I had been using. I've since uninstalled it completely, and added the noscript add-on to FF.