What is this blue button next to URL in Android Firefox? The blue document looking icon:
It's reader view for clutter free reading, similar to simplified view on Chrome

Reader View is a Firefox feature that strips away clutter like buttons, ads and background images, and changes the page's text size, contrast and layout for better readability. Reader View settings are easily customizable to fit your reading preferences.

Tap it to reveal a blue circular icon with Aa inscribed, from where you can configure the settings for reader view


(Click to enlarge)