I'm having the same issue. "Go To Drive web" opens in Firefox NOT Chrome (Chrome is set as default browser).
Same thing happens when clicking on a ".gdsheet" file in Finder.
Looks like it may be an issue with the latest MacOS update (10.12.5) that is not confined to InSync, but also other programs that link to web interfaces. I only started seeing this issue after installing the update today.
See here for more info:
Looks like we might have to wait for Apple to fix this one.