IMHO worst case scenario = that is normal loads with swap on or off.
Normal user will not see this but heavy users will.
OSX / Apple with 1gb ddr2 800mhz ram + 2.4ghz core extreme will have on avg a 5gb -> 7gb -> 14gb swap if you check the resources and then firefox + me.com =
just add microsoft anything or entourage = it is dead. Just ahd to fix one Apple in the middle of the night
You are not doing to bad.
Some other websites also does this but not as bad as the avg 720mb ram used for me.com ? and having several websites / pages open especially photo's and a mix of this = 1.4gb ram used easily.
All the spam harassment / torture adds on the sides of the most web-pages helps to add to this and with some flash type adds you can even lockup firefox and crash it in any OS but this is old and normal.
Firefox is just plain and simple not capable of running normal user / teenager loads with internet pages / links open in any OS and the current yahoo+google against firefox wars is not helping it.
Terminal if you are running a few installs / compiles then you would start to use swap space as some packages compiling / installing can load a lot during this process. 400mb to 800mb for 1 package I installed yesterday and nothing else running. "emerge"
The combination of buggy memory usage thunderbird and firefox with a few windows open = ram usage hell then add terminal and your results seems normal.
The memory management if there is no swap just forces everything to go starving, run more efficiently and better use their resources if swap is off but will on occasion get full ram use if you go full throttle.
PS> if no swap then linux will use a virtual / temporary swap like feature to cope somewhere in the tmp folders but only as last resort. Swap was there for the small hard drive days to try and limit waisted space to a partition instead of a big blob in tmp folder that keeps on growing for no reasosn.
To test this use vmware with 1gb ram and 2gb swap and see how many times you can crash most linux distro's with installing packages and not using firefox root terminal only no desktop env.
I hit 3 times a hour in the last 2 days I tried using gentoo.
"linux = gentoo based in my concept" i486/x86 as base copes better / more bugs ironed out as compared to i686 on most kernels.
Hope this helps shed some light on things.