First of all,
Welcome to the forums!
Second,
Why are you using flash objects? That is wayy to much overhead for something like this, and truthfully, not needed
at all.
I would use a animated GIF for something like that. Flash is not needed because it's just a mouseover. You can create two different images, like you have already done, and just programatically put it on the site with JavaScript.
BUT! If you want to keep the flash.. for whatever reason
.. then check it out:
You aren't using the width and height properties of your embed and object tags. You need to include those in order to make sure it will be outputing correctly on all browsers. Firefox uses the embed tag, so make sure those two properties are specified on there, and I would do it on the object tag as well just in case.