After much testing I've found that CoComment is the culprit. For quite a while now after I've used my Firefox web browser for 3-4 hours, my browser slows down to the point of not working. I then kill it. Even though I have set a maximum cache, Firefox then goes on to exceed this cache by about 50%, slowing down my system until it finally annoys me enough to kill it.
I thought it might be Simple Delicious, a Firefox browser plugin to handle Delicious social bookmarks, but alas, I was not correct. I chose Simple Delicious because the Delicious plugin was a pig and took over parts of my browser. I do detest invasive plugins. After all, it is my browser.
Even when I simply leave my browser active but not use it, the browser gets fatter and fatter until it can move no longer. It sounds to me like a software memory leak issue. The software uses incrementally more and more memory without an adequate release of memory it does not use, thereby resulting in bloat.
How did I conclusively know that it was CoComment? I disabled it and was able to use Firefox for a whole 8 hrs without it slowing down and dieing. It's too bad, too, because CoComment is really functional. It tracks my blog comments. Great concept, bad implementation. I'll be looking for a replacement. In the mean time, when I want to comment I'll enable CoComment, restart Firefox, do comments, then disable CoComment, then restart Firefox. It sounds just as annoying as I wrote it.