We've built Firefly from the ground up with complete browser compatibility in mind. We support cobrowsing in all current versions of major browsers, and many versions of older browsers.
We even have full support for Internet Explorer 9+ and the iPad.
Different browsers render the same pages in conflicting ways. If your customer is on an iPad and the customer rep is using Internet Explorer, they'll be looking at two slightly different pages.
In the past this was a problem for cobrowsing software, as customer reps could never be sure if they were highlighting the right thing on the customer's page.
Firefly doesn't have this problem. We allow reps to highlight elements on the page which are accurately rendered on the customer side 100% of the time.