> If you configure Firefox to not automatically update, and then use, say, a package manager to update it, automatic updates will get re-enabled.

That isn't what the bug report says - package managers don't use the Firefox installer. Are you talking about a real bug (if so, is it logged?), or are you spreading FUD?

Depends on the package manager. On Windows both Ninite and Chocolatey use the Firefox installer and re-enable Firefox's automatic updates.

On the other hand, Firefox does respect locked preferences after update. Debian sets both app.update.enabled and toolkit.telemetry.enabled locked to false.