Working on WebGL at Mozilla, I can second that open source drivers are inherently much more developer-friendly than closed-source ones. Any driver will crash, and work-arounds will have to be found, and open source drivers give right away two large advantages: 1) good stack traces 2) the possibility to look at the code.
I'd also like to add that open source drivers are the ones that have seen the most rapid quality improvements in the recent past. All the significant bugs we file against drivers @freedesktop.org get fixed within a few months and due to the quick distro updates, bugs don't stay around in the wild forever like they do on other platforms. That's why desktop Linux is now the platform where we see the highest WebGL initialization (blacklist) success rates, above 70%. This is in sharp contrast with the situation just two years ago when the idea of running WebGL on open source drivers was unrealistic.