My favorite feature is the reader view (does not always work but if it does it shows a little book in the URL bar), that removes all clutter and interstitials and just shows the main text body of the page in large enough automatically wrapped text.
Reader View is unfortunately no longer being maintained since Mozilla decided to integrate Pocket. It bugs me since I find Firefox's Reader View actually does a better job than Pocket in most cases of stripping out the clutter.
Pocket were added as it were easier than implementing their own Reading List, although Pocket will be shipped as a default extension in the future rather than being integrated.
https://bugzilla.mozilla.org/show_bug.cgi?id=1215694