Why a different distribution mechanism

Want to be clear about "allowing remote code to take control of my desktop" - what happens is that when you click on the website to install an extension, this triggers the downloading code (not in the same process as the browser plugin) to put up a dialog asking if you want to download and install an extension. So, if you consider a MIME-type helper secure, you shouldn't be too afraid of this. The advantage of a plugin, beyond slickness, is that we can do additional security checks to avoid users being tricked by arbitrary websites into installing extensions. Debian of course, is free to package the browser plugin as a separate subpackage.