There's 3 things that will contribute to FF's memory usage. Extensions, History, and Back History. IE's affected by all but the extensions, since IE doesn't use extensions.
Naturally, the more extensions you load in FF, the more memory it's going to require. That's the trade-off if you want a lot of functionality built into FF, beyond basic browsing. Also, some extensions do cause memory leaks. Adblock used to have one when blocking images that are set to reload/refresh automatically, basically each time the image refreshed, adblock took another chunck of memory equal to the size of the image being blocked. From my understanding, it was fixed in the latest version of Adblock, and has been fixed in Adblock Plus for a while now. But it just goes to show that even some of the best extensions, regularly maintained, (not some random dude hacking away in his basement for some obscure extension that only he and maybe one other person in the world would use) could be causing problems.
Extensions I use on every install of Firefox I perform:
That's all that's really needed, though I also install "Tab Clicking Options", "Tabbrowser Preferences", "Download Manager Tweak", "DownThemAll", and "IE Tab" on my personal machines (IE Tab only on my windows ones since I don't actually have the IE browser installed. It's nice on the rare occasion I'm faced with an IE only site ... which isn't very often. I try to avoid them.)
The History issue, as I stated above, can be fixed easily by setting your History to 0.
Back History is a little trickier, and was introduced in FF 1.5. Basically FF stores each page you visit in it's back history (hit the back button and it goes back to the previous page), same as IE. The problem comes up, when you have extensive back histories (by default FF will store something around 8 pages in it's back history), across multiple tabs (each tab has it's own individual back history). There's no gui switch to change this setting, but there is a configuration setting that can be changed. Most people never go back more than 5 pages, so 8 is a bit excessive. And actually, the number of pages FF goes back is scaled up based on how much memory you have in your machine. The more memory, obviously, the more pages it will store in it's back history.
In the address bar, type "about:config" without the quotes, and hit enter. This brings you to the "hidden" configuration of Firefox. Hit Ctrl-F and search for: browser.sessionhistory.max_total_viewers
If you're using 1.0.7, don't bother, it won't be there. It's only in 1.5 and above.
By default, it's set to -1, which tells FF to scale based on system memory. If you set it to 0, it will disable the back history cacheing. You'll still be able to use the "Back" button, it's just that the page you're going back to isn't stored in memory, so it will have to be re-rendered. Again, not really a problem if you're on broadband. This was another "feature" added to speed up browsing for people on dial-up. If you set it to 1, it will only cache one page back, and subsequently any number higher is how many it will cache back, but the trade off is each page takes up memory, so the more you cache, the more memory is going to be required.
I set mine to 0. I rarely go "Back", as I usually open all links (including forum posts/threads) in a new tab, and close the tab after I'm done with it. But the times I do go "Back", I've not noticed any adverse behavior by having the value set to 0.
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