Mozilla Add-Ons Blog, 85% of Firefox 4 users have at least one add-on installed. The average user has five add-ons installed. The figure doesn't include the Personas feature and excludes add-ons bundled with other software that users haven't actively chosen to install.According to a post on the
"We previously estimated that at least a third of Firefox users had chosen to install an add-on, but knew the number was higher than that," wrote Justin Scott, the product manager for add-ons at Mozilla.
I don't have any stats handy, but I suspect the numbers for Internet Explorer will be very small. In an interview with ReadWriteWeb at MIX, VP of the Internet Explorer team Dean Hachamovitch told us that Microsoft's research indicated that only a tiny minority of Internet users use browser plugins.
Mozilla didn't say how many Firefox 3.X users are using add-ons, but given the quick adoption of Firefox 4 and the current estimates of overall browser share, I think it's safe to say that browser plugin user have graduated from tiny minority status to significant minority status.
Of course, Firefox 5 was just released yesterday, so who knows how that will change the equation.
(Hat tip Mike Loukides)