Not keeping your browser plug-ins up to date can be dangerous. Security holes show up in Flash and Adobe Reader on a regular basis, and exploits sometimes allow for remote code execution. That's presumably why Mozilla is going to start nagging Firefox users with notifications about out-of-date plug-ins. Here's the spiel from Jorge Villalobos, Mozilla's Add-ons Developer Relations Lead:
Firefox users who have outdated versions of the most popular plugins will soon see a notification urging them to update when they visit a web page that uses them. Old versions of Silverlight, Adobe Reader and Adobe Flash on Windows are covered by this.
While you are free to ignore the warnings and continue using your old plugins, we strongly recommend that you go to our Plugin Check page and update them as soon as possible. Old plugin versions can cause stability problems and are potentially insecure. Keeping them up to date will ensure that you have a great Firefox experience.
I find update reminders as annoying as the next guy, but this seems like a necessary evil. Too bad the browser can't update plug-ins silently by itself.
Personally, I like the way Chrome handles things. Flash is built-in and updated silently with each new Chrome release, while PDF support is native and doesn't require Adobe Reader. That doesn't cover Silverlight, but the majority of exploits out there seem to affect Adobe plug-ins, anyway.