But do you believe that more than 1% of web browser users install and use one or more browser extensions?
In 2013, this article[1] gave numbers of anywhere from 9% to 22% of users blocking ads, which requires an extension AFAIK. So, "yes". Even if those numbers are exaggerated due to skewed measurements, I'm pretty sure it's going to be more than 1%.
Edge supports Flash already, so if that's all that 90% of people use, it won't be a problem. And it's typically categorized as a "plug-in" rather than an extension, so I wouldn't lump it in with the extension functionality.
Not sure if it has Silverlight, but since we aren't streaming the 2008 Olympics and Netflix doesn't need it anymore I'm not terribly worried.
But do you believe that more than 1% of web browser users install and use one or more browser extensions?