Review for Larger Message List by satbunny
Rated 3 out of 5 stars
Sadly out of date but the effect can be easily replicated:
Open the Menu and go to Preferences > General. Scroll all the way down and at the very bottom, click the Config Editor... button (accept the warning).
You're looking for layout.css.devPixelsPerPx, you can just search 'pixels' and it'll show up.
Click on layout.css.devPixelsPerPx and edit the default value of -1.0 to a scale factor you are comfortable with (example: 1.5, 2.0, 2.25, etc).
Clicking OK will allow you to immediately see your changes so you can dial it in pretty easily.
To create your own collections, you must have a Mozilla Add-ons account.