Worked as described, but unfortunately, it's not useful to me, as it doesn't recognize numbers that are written as text.
E.g. "nine feet", "five inch"

This is quite common in books and stories, such as on royalroad.com.
It's hard to get a feeling of size in those stories, when they are written using imperial units.

While the amount of work the developer put into this addon is commendable, I have to say that using it is just too much hassle. There is no possibility to set automatic conversion of all pages. The "SI" button that shows on the address bar intuitively should work as a conversion button of the currently displayed page but it does not. The only thing that it does, is being a way to show a right-click menu with several options. I mean I think it does that because in the documentation the developer gives a load of useless (to the end-user) information about how exactly the conversion works (why do I need a diagram comparing pitch sizes in various sports?) but no hint on how to make it work. What does the red "SI" mean? What does the white "SI" with a black border mean? No clue. At least now I know a rugby pitch is wider than a football one.

Is there a feature to auto-convert all pages?

Looks very well thought-out and promising, but sadly it just doesn't work very well. It fails 90% of its own tests, and fails to recognise very common units such as ft3 for cubic feet, so that "22ft3" is converted to "6.8 m". There's also absolutely no contact info anywhere, so it's impossible to report issues to the author.

Yes, it's early days, as the author clearly states. But well thought out, well done, and - above all - incredibly needed.