HTML mode is only supported in IE. The other modes (Win, AA, Region) are available and you can use them even though Windows mode does not reveal much in case of Firefox. You could also use startup parameters to make your life easier. I quickly googled the following link:
http://kb.mozillazine.org/Command_line_arguments
For your purpose, this might be sufficient.
You will not be able to migrate your solution to IE later on, you will have to create a new object and actions for IE.