I've got Firefox on Vista that's been running for 24 hours(ish) with a script reloading a few of those every 5 minutes.
Task manager claims a private working set of 91MB with 16 tabs spread across 2 windows with a whole load of extensions (adblock, firebug, web dev, etc), in a box with 2GB of RAM - it's also worth noting that Fx will use a varying cache size with the amount of RAM you've got.
Given I've got 46% free, it's hardly slowing my machine down - it's the cases where it seems to go loopy and gobble 900MB on a machine that only has 1GB that are the issue here...