There would still be the problem that someone would have a log of your IP address downloading a Firefox update, but it wouldn't be Mozilla. Also, with a big enough list of mirrors, across 100 different jurisdictions, the probability of any given mirror being subpoenaed and having data on any given user is very small.
As for how to bootstrap the whole thing by letting the user securely download Firefox in the first place, that is of course difficult, but it should be an infinitesimally rare event compared to checking for browser updates. A user could therefore take special care to use public Wi-Fi when doing the initial download.
"Best practices" are moot unless they're implemented, and since the default is to log these things, then something (even a blog post from them claiming it's been done) has to exist to show it's there and doing the right thing.