All of these "low-end" operating systems should have at least 1 GB of RAM as the bare minimum, and something with a more modern OS like Ubuntu or Windows 7 should use at least 2 GB of RAM, otherwise you have the same problem.
Interestingly, #3 is the home page of an old blogspot.com blog. It's mostly just text and a few inline images, and it still somehow manages to occupy 132 MB of RAM.
Firefox is lighter on the RAM usage these days especially with more tabs because of the sandboxing.
About:Memory in Firefox right now, with nothing other than this reply box open, shows JS using 67.29MB and well over 100MB of memory used for other purposes.
(Likely this is left over from tabs I have closed)
(Likely this is left over from tabs I have closed)
I think it's just general overhead, since I have similar figures when starting FF fresh with no tabs.
If you look at "window-objects" under explicit allocations, you can see the memory for individual pages; the page of your parent comment weighs 2.2 MB for me.
But that's without history... when you click links, it will keep the previous pages cached, and I think that is what can really eat a lot of memory real quick. But I would also hope that memory gets released without much of a fuss? I don't know that, but if the RAM is made available when system resources get low, I'd say cache away, since unused RAM just eats electricity for nothing :P