about:support should list some statistics about the user's experience with Firefox's performance. Johnathan suggests starting simple with privacy-insensitive, coarse stats like time to browser load, time to end of session restore, average page load time, etc. about:support is relatively simple to change, easily accessible to the user, and SUMO people often ask users to paste its contents when diagnosing problems.

Related to performance, I would be very interested in knowing if our accessibility engine was instantiated or not. This could be gathered programmatically, and included if the user agrees. Getting more information about why our accessibility engine is invoked and how it impacts users is something I need. Aside: I found this bug when I was about to file a request for an about:performance, which I ended up not filing.