There were thousands of changes from Firefox 58 to Firefox 59 (I count > 8900 changesets, though some are merges).

The release notes only list the most visible and most unexpected changes. Ongoing memory and performance work rarely clears that bar. It's happening all the time.

They are doing that, with the whole Firefox Quantum thing. I see no need to mention less RAM utilization now given that they reference Quantum. These aren't meant as an advertisement, they are giving the changes since the last update, so there's nothing to say about it. These release notes aren't meant as advertisement.