From what I can tell Sigil creates an html page and sends that file to the vaildator....at least the address shows:
when I click on Validate stylesheets.
I have never set a default web browser within Sigil - and don't see a location that would let you set one.
So, I would think that it is just trying to open an html page and relying on your system defaults to open a browser.
You may need to go into your OS settings and (re)select a default browser. (Right click on an html file, select open with, and select default when using windows)
The next thing I would try is making sure you clean out any addons/plugins/bars - I've had those cause cause conflicts in the past.