It should be there - Have you rebooted since installing if not do so ?
If you have rebooted, check that it isn't hidden, the little inverted triangle on the taskbar (right side of it), hover your mouse pointer over it and a tooltip should display Show hidden icons. If it is there just drag it down to the other tray icons and it should stay there.
Your firefox is way out of date, there is 4.0.1 being the latest version 4.x incarnation and only just released firefox 5.0.