Something that seems to improve Firefox's performance (and hence the rest of the applications) is to clear out the Downloads list periodically.
I also notice the OP is running a fair amount of "standby shit" which consumes memory allocation (it always annoys me when Quicktime, OpenOffice, etc doesn't *ask* me before installing not-necessary 'quickstart' resident crap in my system tray...
It may also be that the emulator you're running doesn't manage its memory well. Sometimes programmers don't remember to write their code so that it releases memory back to the OS, creating what is called a "Memory leak". Eventually the OS has no more memory to offer other applications because the errant program has it all allocated (even if it isn't actually using it).
A reboot clears the problem up til the next time you run the leaky program.
Games most frequently have this problem but it can be an issue with any incompletely tested software.