BeeLine Reader is an add-on that makes long paragraphs easier to read through clever coloring of the text. I learned about it last month, and after a few weeks I'm hooked. The creators hope to help users with dyslexia and ADD, but even as someone who does not have either of these, I still find the add-on makes reading more pleasant.
It's easy to install and use on Firefox for Windows, Mac, or Linux. Unfortunately, it's difficult to get the add-on working in Firefox Mobile on Android. I've been able to get it going on some non-Firefox browsers, but even then the add-on doesn't work with 100% of its features.
It would be great if Firefox for Android had this as an available add-on, or at least the codebase were fixed so that the add-on could be installed in Nightly or Fennec. (My attempts to use the add-on in Nightly, Fennec, and Mull are chronicled this blog post. It can be installed, but some unknown error prevents it from coloring the text.)