Firefox (at least on Windows) has a special Flash installer baked into it for several years -- if it can't find the plugin, it prompts you with an infobar, which then kicks off a streamlined installer that downloads a xpi package and installs it even without administrative privileges.
When they first implemented it, they got a special license to distribute the Flash xpi from addons.mozilla.org, and did so happily. Mozilla has absolutely distributed the Flash binaries themselves before, but I think the file is hosted by Adobe these days.
Fairly recently they baked in a special Flash updater too -- if your Flash plugin has known security vulnerabilities Firefox will prompt you to automatically update it: http://blog.mozilla.com/metrics/2009/09/16/helping-people-up...