Paul Bausch is absolutely right that Firefox’s current update notification, a popup dialog that gives you the choice between “stop what you are doing and restart your browser right now” and “be nagged at some random later time until you stop what you are doing and restart your browser” is the sort of annoyance we use Firefox to avoid, not to seek out.
But the previous version, a red Christmas tree that I think was supposed to be an arrow displayed up by the throbber, flat out didn’t work. For every single non-geek on whose computer I installed Firefox, that was something for me to see when I was doing something else for them, my little Christmas present of also getting to update their browser, and not anything for them to even notice, much less do anything about.
A couple of hours of thinking about less extreme methods of reminding people about an update without being invisible has only left me reinventing Clippy. So, what sort of notification can’t be missed, but can be ignored until you’re ready for it?