Maybe try another browser? I would suggest giving Falkon a try. Though it does not have all those fine-grained configuration tweaks the Firefox about:config has.
If you can tell the browser where to cache the files, or at least find out where it does so by default, you can cache to a tmpfs. /tmp is usually one, or you can mount your own somewhere, even on top of the default browser cache location. Though it means that the cache will be gone after reboot. And that the RAM might fill up if you do not limit the cache capacity strictly enough.
Thanks Kevin, I started looking into attempting the same with chromium and found allocating its cache to tmpfs /tmp worked there. I also upped to original tmpfs /tmp allocation from 20M to 2g, and now Firefox is happy.