Replacing the call to 'sensible-browser' by 'xdg-open' in ubiquity would probably work, because if BROWSER isn't set xdg-open falls back on a hardcoded list of well-known browser names, and that includes firefox.
Replacing the call to 'sensible-browser' by 'xdg-open' in ubiquity would probably work, because if BROWSER isn't set xdg-open falls back on a hardcoded list of well-known browser names, and that includes firefox.