Well it detects IE and Firefox on mine in the Custom section, not Avant or Maxthon so MS is a little picky. However when you open this window the option should be Use my current Web Browser which is ticked.
Now if you open the Non-Microsoft section, it shows Firefox as my default browser, which is correct as that was my current browser.
None of this should matter providing you have Opera set as your default browser and it works, opening html/htm files or web URLs using Opera.