Plugins are the only reason I use Firefox over any other browser.
Can you explain to me why Firefox would be better off if I didn't have Rikaisama for my Japanese studying or one of a number of Youtube plugins that make the site bearable again by forcing annotations off and 1080p on.
This is most likely an addon, a Firefox extension written in (mostly) JS and XUL. Plugins are the external runtimes embedded in browsers with NSAPI &al: Flash, Java, Silverlight, etc...
a lot of modern firefox plugins are already written this way, but there are probably a significant amount of legacy addons that would need to be rewritten