Mozilla has the release notes available as well, which tout Phoenix’s “customizable toolbar” and “reasonable default settings.” The FAQ is well worth a read, with gems like this one:
You said this is a lean, lightweight browser, but it’s 8MB! I laugh at your silly lies!
Take it easy, sport. Phoenix has many files that override those in Mozilla, but it also has a new set of files. These files render a ton of files in Mozilla unnecessary, but we haven’t yet stopped packaging the old files. It also still contains the modern theme and all the composer UI. In short, we haven’t done any work yet to minimize the size, but we expect to be able to hit 6MB with a little work.
Remember when a couple MBs of disk space mattered? Simpler times, those.