While this addon works in most situations, I've encountered pages that somehow stick to a 'minimum' width, so that what this extension does is merely changing the font size. Microsoft Edge does not have this issue. When I dock my browser window to half of the screen, Edge displays the content without the horizontal scroll bar, while Firefox with this extension set to auto-fit, still does. This is an example page that shows this issue:
https://bbs.saraba1st.com/2b/forum-51-1.html

Hope this is not limited by some Firefox intrinsic stuff and can be fixed.