Flash is one of those things that comes heartbreakingly close to actually working, particularly in a 64-bit Linux OS. There’s an intricate scaffolding that plugs the 32-bit Flash code into the 64-bit Firefox superstructure, but …
Worse, there’s an error in the Hardy Flash 10 repository entry that points to a nonexistent Macromedia download website, so all the usual hints & tips don’t work. That seems to have broken in December and the automatic installation fails quietly. Using sudo apt-get update flashplugin-nonfree reveals the problem.
Unfortunately, the sum total of all my fiddling was a Flash installation that sorta-kinda worked, with a significantly flaky crust.
What works slightly better: force the version to Flash 9 by pinning the flashplugin-nonfree package version in apt-get. Start by removing everything related to Flash. Then, with a clean slate, this post on the Ubuntu forums shows how to get a clean installation:
1.) Create /etc/apt/preferences with this entry:
Explanation: Flash plugin from hardy-backports was broken on 12/24/2008;
Explanation: pinning to hardy-updates for now until it is fixed
Pin: release a=hardy-updates
2.) sudo apt-get update
3.) sudo apt-get install flashplugin-nonfree
4.) Restart Firefox
It still gives me a blank gray background where the Flash should appear, which is sometimes cured by reloading the page, but it seems more stable.
Adobe has a genuine 64-bit Flash alpha out, but (as of the 16 December 2008 version) it reliably crashes on sites I really care about. Like, for example, nytimes.com.
Memo to self: un-pin this in a few months and see what happens.
Update: It still jams up with gray screens where Flash should be, no matter what I do.