Here's what I did :
Move firefox-installer to /usr/lib (I renamed it mozilla-firefox so the mplayer plugin would install properly)
Then using KDE:
right click on the task bar, choose add application button (non kde)
under path to executable type /usr/lib/mozilla-firefox/firefox choose an icon (kde has the firefox one already) You can drag and drop the icon to the screen if you don't want it on the taskbar, or use the KDE control center and follow a similar procedure to install the icon. Not familiar enough with Gnome to post about how to do it, but I'm sure the procedure is similar. Putting the directory in /usr/lib makes it available to all users.