Two things I appreciate here:
1. Google recognizes that browser extensions are a geek perk, not something with broad-based appeal. I imagine most Firefox users don’t even know extensions exist. And I imagine even fewer people know Google *has* a browser. All this may change in time, but right now it’s not an issue.
2. I will be stunned — jaw-droppingly stunned — if Google clamps down on ad blocker extensions for Chrome. That would be so insanely stupid. Like, Facebook/Beacon stupid.