It could be a problem with someone with an old machine, like me. I have 512 MB RAM, which good enough for me. It may be a case of habit, or what, but I have never seen any browser sucking up more than 140 MB.
I don't know if one should combine firefox.exe with plugin-container.exe. My 3.6.16, would be using a total of 110 MB. I, rarely, keep more than three tabs open, at a time, nor use a single browser over 30 min.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:18.104.22.168) Gecko/20110319 Firefox/3.6.16
A minority may be right, and a majority is always wrong
~ Henrik Ibsen
WinXP, SP3, 512 MB, SM2.17.1, FF21, TB17.O.5, IE8.0, Google Chrome 26, Ghostwall , Kingsoft5.6, JRE1.7_21.