So, after writing that last post about the audio driver handle leak, I decided to log some data - specifically, the amount of memory Firefox allocates, and the number of handles in the Symantec Anti-Virus process smc.exe. It's now been about a week since I started gathering data (although unfortunately the power went out in the middle, so I ended up with two smaller replicates).
The data for smc.exe shows that it begins at approximately 450 handles on startup, and acquires an additional 3100 handles per day (although 'day' is about 14 hours, as I hibernate my computer at night; meaning about 220 handles/hour). This definitely doesn't seem normal, and I'm going to venture a guess that it's a handle leak. I also noted that the increase seems to be linear over the course of the day, so is unlikely to be related to something like automatic update.
I already knew that Firefox was hemmorhaging memory. If I recall correctly, the amount of memory allocated by Firefox increased by 200-300 MB per day. This time, I tried using Firefox for several days without two of the three addons I normally use (the third was NoScript, so I didn't want to try without that unless I had to). While this test didn't last as long as I'd hoped (thanks to that power outage), after four days, Firefox had only increased from 125 MB (when I first started it, with a lot of saved tabs) to 205 MB (now). In four days I would have predicted it would hit 600-900 megs.
This strongly suggests that one of the two plugins is responsible for the massive leakage, although I'll have to watch what happens after I reenable the one most likely the be causing the leak (as the other is newly installed, and this problem has been around for longer): Feed Sidebar (version 3.1.6). So, we'll see what happens with that. Might have an answer in another 4-7 days about that.