Firefox made it dead-simple for people to switch from IE to FF. The whole install process for Firefox takes maybe 120 to 180 seconds, from the time you click "Download" to the time you are up & running. And Firefox imports all your IE data in one click, and also recognizes common IE shortcuts. Ctrl-E is IE's shortcut for the top-right corner search box -- it also works in FF, try it!
The reverse is not true. It's hard to switch from FF to IE. IE takes 10-15 mins, plus a restart to install (at least it did when I installed IE8 it a couple months back). It doesn't let me import FF data, and it doesn't recognize FF shortcuts. By making it so hard to switch to IE, they have basically made it impossible to grow IE's market share. Is it any surprise then that IE is dying?