IMac getting slower and slower

I have an Intel iMac and it was really fast when I got it. In the past couple of months it's gotten slower and slower. At times, it's so slow that it is painful to use. Two things which could be causing the slowdown are not enough disk space and the fact that I don't restart the computer very often, but even when I make sure I've got plenty of free space and I reboot, it does not seem to make much of a difference. The biggest virtual memory and CPU hog is Safari, so maybe if I switch to Firefox that might improve things.