Does the job and works reliably. Easy to use - click on icon to enter and exit. No 3-finger salutes on the keyboard or getting stuck in reader mode requiring a total refresh of the page.
Operation is MUCH more predictable than the "more popular" reader that's just named "Reader" for FF57+ (which also appears to collect usage data).
All that said, I like the "Enjoy Reading" add-on for the PaleMoon browser a bit better, but give this a try for FF57+. YMMV