Firefox

Same, although from my work I also know that it's entirely impractical to do opt-in if it is about user interaction data. You need specifically the data of those users that neither interact with opt-out nor opt-in prompts, meaning you get their data if it's opt-out but will never get it when it's opt-in.

The reason those are the important ones is that your power users will already tell you about their interactions and ideas and ires. Metrics for them is interesting, but not actually as required. You need the data of the (silent) majority to judge how your application is actually being used by most.