Rated 5 out of 5 stars

It definitely works, but most font sizes aren't changed (for example, the text box for writing this review has a changed-size font, but almost all the other text on the page including on the add-on download page, and my review on the "read reviews" page, isn't changed). It changes the colours for everything in Firefox and carries on for as long as you have the add-on toggled, so you can move from page to page without having to re-click it. The settings box to change the font size and colour is in your normal firefox settings page, the problem is that the colour preferences box can't scroll, so in my case the settings for "link colours" are cut off and I can't change them. Still, it works perfectly fine, I only wish you could add it to the context menu because that's much easier for me than clicking on the navbar.

EDIT: Okay, thanks for the reply! Right now I'm using another addon that changes the minimum font size using a keystroke, it works really well.

It does not change the font size, just what font is used (which might be different sized). What you might want is a button to change what is the default minimum font size. I have already added that to the next version.
I will consider adding it to context menu for the next version.