Are you saying that Firefox, once it reads its cache from the HDD, doesn't keep in it RAM, but Opera does? I assume Opera saves the cache to HDD too...
And I prefer this fix over any other browser any day
Well I am not really exactly sure of all the changes, but I can tell you what I think is correct.
Firefox use to cache stuff in RAM you were currently still viewing. It would move it to HDD when the site was either closed, navigated away from, etc. It also use to pre-cache stuff into RAM from your homepage to ensure the browser loaded faster. These features were both removed. Thus required increased accesses to the HDD during use. This made Firefox starting with 3.25 (I think) rather sluggish compared to 2.0.8 up to 3.0, but it used less RAM to run which was the goal I guess.
Then for me personally, there was a memory leak in 3.0 and the first update to it. As I opened and closed tabs, the RAM it sets aside for use was not released at times. Eventually it would reach something around 1+ GB of RAM being used for just it. The more RAM it had, the slower it got. It would also occasionally just take 44% to 50% of my processors power for no reason. It would just hold the CPU there forever. I could close it, and would keep holding the CPU cycles until I force closed in with the task manager. While the memory leak was fixed in 3.5, I still have the processor issue. While it didn't really bother my system, I would forget to forcible close it at times, then try to play a game and it would piss me off.
Opera works off the old Firefox system. It eats more RAM to use, but it is much faster and responsive. I also like several features being built-in that I use to add with add-ons. It also has some of Netscape 7.2's customization options, and properties layouts. Anything that reminds me of Netscape 7.2 is awesome in my book.