I dunno, I am not too quick to jump on blaming the browser as "memory leaky". I am willing to bet some sites are not built with your memory in mind. Consider sites that just load and load javascript objects ad nauseum. Should the browser say : "ok that's enough web site, no more java script objects for you" ? and if it's a reasonable X for a single site ... and you have 300 sites open .. is 300X ok for your computer?
This is not a simple problem, so I think it's ok to ask a user to "be considerate" of their own memory/ system resources (heaven forbid the user knowa a smidge about how her/his computer works )
David