How can I specify a browser that GWT tests should be run on?

Question

I am writing a GWT project that uses HTML5 canvas.

To test application I use GWTTestCase.

For firefox some functions are supported since gecko 1.9.
While testing, firefox 3.0.1 is being used and errors occur.

How can I set up GWT tests to be launched on firefox 3.6?
(in Eclipse using GWT plugin or using build.xml for ant)