I normally would think maybe this was "just me", but the satellite installation dude was here yesterday and was complaining about the following error, so now I know it's something in the wild. Occasionally, and quite often lately, searching on Yahoo! returns the following 999 Unable to process request error message.
Since the satellite install guy has been seeing it as well, and he very likely doesn't have the same Firefox extensions as I do, I'd say there's a darn good chance that it's not an extension causing the problem. However, the error does mention unusual activity by the ISP, so maybe that's it, although it's weird that it would only happen with Yahoo. So, Yahoo, y'all sleepin' over there? Wake up! Get your elbow off that button! It's causing a problem.
Problems seems to be resolved. My new satellite provider, Hughesnet, didn't give me a static IP address at first. The dynamic ones I was getting on a rotating basis must be part of a list that Yahoo blocks as abusing their engine. I got a static IP today, and that appears to have solved the problem.