I now have Firefox and Opera installed on my Desire. I thought it would be interesting to run the Acid3 test against them, and the standard Android browser. The result was 97/100 for all three browsers, and whilst not top marks is still pretty good. Overall, Opera was fastest, but the on screen rendering looked a bit odd. Firefox and the standard browser gave very similar results, though I felt the image was slightly better quality in Firefox. So, overall, not much to choose between them, though the Opera rendering speed was impressive.

Try the test for yourself here:   http://acid3.acidtests.org/