How to install Flash for Firefox 3 beta in Ubuntu
First make sure you have the Adobe Flash plugin installed. It is not packaged with any version of Firefox.
$ sudo update-flashplugin
$ sudo apt-get install flashplugin-nonfree
This package of flashplugin-nonfree was made before the Firefox3 pre-releases, so we have to add it to the libraries manually. This method will “link” it from the default install location (not just copy it) so in case you upgrade flashplugin-nonfree in the future, it will be updated for Firefox3 as well.
$ sudo ln -s /usr/lib/firefox/plugins/flashplugin-alternative.so /usr/lib/firefox-3.0b3/plugins/flashplugin-alternative.so
If you are using a different version of Firefox3 than I am (firefox3.0b3) you’ll need to modify that line. To test if it worked go to a flash site such as YouTube.com.