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Once you've upgraded to Jaunty and done the mandatory reboot, you're likely in for a big surprise -- Flash stops working in Firefox. This is related to a bug in Network Manager which drops the connection during the upgrade. Since Flashplugin-nonfree is really a dummy package which downloads the Flash player from Adobe, the download fails and the package shows as installed when there really isn't anything there.
The solution? There are two, one significantly better than the other.
- Reinstalling flashplugin-nonfree will solve the problem short-term, but you're still stuck with this wonky download behavior which also breaks the package whenever Adobe updates Flash and creates a binary which doesn't match the package's checksum.
- The better solution is to uninstall flashplugin-nonfree, enable the Partner repository, and install adobe-flashplugin instead. Flash is actually in that package, so you won't have any strange failures.
- Of course, you could always install the plugin manually into your $HOME/.mozilla/plugins directory, but then you'd have to manually upgrade Flash, and it being the security nightmare that it is, there's almost no circumstance in which you want to do that.