I read today in Lifehacker about disabling Firefox’s disk cache and increasing the memory cache, as an alternative to putting the disk cache on a ramdisk. The trick can work, depending on the types of sites you visit. But the two aren’t quite interchangeable. The disk cache stores compressed images and (I believe) html. The memory cache stores uncompressed pictures for fast rendering, and no html. Content stored in one isn’t necessarily stored in the other.
Increasing the memory cache can indeed be beneficial, and disabling the disk cache can increase SSD life expectancy. It’s worth a try, but not universally beneficial. But on a high-memory system (2 GB or more), increasing the memory cache is almost always a good idea. But a high-memory system (and I’ll allow that that may be a system with 3-4 GB of RAM, minimum) benefits at least as much from putting the disk cache on a ramdisk.