As Tobiking says, the teleporters will only teleport you to another teleporter if there is another one present in the direction you entered it. If you enter it and there is not, it will just disappear. I will try and make this better explained in the help text.
I've installed Firefox 126.96.36.199 with java 1.6.0_05 on another Win XP pc, and it works well, so I can't see what could possibly cause the problem with not being able to move? Perhaps it's the keyboard driver or something else. Hard to figure out.
*update* I've made the game so it is in focus all the time except when the Help window is open. This should help with the Firefox issues. Also I've made a sound for the disappearing teleporter (if there is only one) and explained this better in the Help window.