Only a three star, because yes, it does convert to metric. The problem I have is that I have to press the button to do so, I want it to be fully automatic every single time I enter a new website for a new recipe. I also want it to remove the imperial units, not have it be 500 cups (2000 grams) of butter.

Also, I did add 2000 grams of butter to my cake and now my oven is filled with butter and I can't get it out. This is not relevant to the review I just deeply screwed up.

Perfect for cooking as a Brit - thanks so much!

It works just fine for converting US measurements to metric, but I need the opposite. Someone please work on that for us.

Does not convert inches to cms.

Not just an excellent simple converter, but will attempt volume-to-weight conversion, e.g. 1 cup of sugar -> 200 g sugar. Love it!

Does what it claims to do, what more can one ask for?

Great idea, but what we need is an extension that will convert metric units into standard US sizes. The ones that used to do so no longer work on current Firefox builds.

I don't need to go to weight conversion websites anymore! (http://www.kingarthurflour.com/learn/ingredient-weight-chart.html is a good one, by the way.)

All of the weights I need are right there on the page, this is wonderful. It does not take 20 seconds to run on my machine, more like one or two seconds like the developer reported.

This Add-on really changes the text of the recipe, by adding metric measurements between square brackets. It will take some time (like 20 secs?) but then it will even take into account the mass density. I checked some of the conversions, and saw no mistakes. Awesome!

*Edit: I cannot add an answer to your question, Falk Hüffner, so I add it here.

I randomly tried the add-on here:
http://www.foodandwine.com/recipes/minnys-chocolate-pie
Today I tried again, and it took me 26 seconds.

My computer: a Toshiba Satellite L350D laptop
Operating system: Linux, Ubuntu 14.04 LTS
Browser: Mozilla Firefox 48.0