I use this feature all the time when I want to quickly scan through an article online (I generally push everything I want to read in detail to Reader). Most browsers now include a reader view (or at least an addon / extension you can install), so you don’t need to use Firefox for this.
It’s especially useful for pages where the text runs on too much (like in this example).
For the most part, I prefer reading light text in a serif font on a dark background. I also increase the size of the text, compress the margins, and reduce the spacing between lines. This is a personal preference and I’m not suggesting it’s better than other formatting options.
The Reader View in Firefox also strips out ads in the page (if there are any) as well as other distractions.
Find out how to use Reader View here.