Recently, Abelardo Gonzalez produced a font called OpenDyslexic, which is designed to be more readable for people who suffer from dyslexia. It’s free-as-in-freedom, so it’s OK to just download it and use it in any way you want.
It’s possible to set this font (or any font) as your default font for reading web pages in Firefox, and even to override the font choices websites make so that things always display in the font you choose.
If you would like to do that, you need to install the font in your operating system by downloading this ZIP file and double-clicking on each “.otf” file in it. This will, I hope, bring up some sort of font installation program in your operating system. Follow the instructions it gives you.
Once you’ve done that, you can set it as Firefox’s default using the lovely instructions provided on our excellent Support website.
I’m not dyslexic so I can’t give an endorsement or guarantee the results, but it seems that it helps at least some people.