One thing that wouldn't be hard to do in theory.
If you have reserved psace on your desktop (like the bottom left/right in flux if your menu is on the bottom) you can edit mozilla and add a button to the toolbar.
This button would resize the window to say 5,5 and put it at 1,1. This is 100% possible using the window class of xul.
It is actually pretty darn easy too

This isn't the solution you are looking for but does offer you a way of pseudo putting it in the "system tray".
If it bothers you to be in the taskbar, you could always use kstart --skiptaskbar mozilla or something to that effect.
Jkrohn