Certainly at the add-on level, as I presume this is how https://addons.mozilla.org/en-GB/firefox/addon/tab-memory-us... is implemented.

If feeding this back to use as a general performance metric, you would have to be very careful to make sure you were measuring the same thing each time which for a complex application could be difficult unless you are only measuring on initial page load (which might not be as useful as you are hoping for). Without this control you would need a lot of results to make any average or other analysis of the metric meaningful.

For controlled tests run by yourself in dev (rather than a performance metric for your app in production) it could be useful though.