[update] This issue should now be fixed! Simply install the package flashplugin-nonfree or search for Flash in Add/Remove Applications to install it.
Can’t get Flash Player working in Ubuntu? Ubuntu’s automatic installation of Adobe Flash Player was broken when Adobe released a new version. The installation will report to be successful, but expanding the terminal shows the problem:
Because Adobe Flash Player can not be distributed, the Ubuntu package for Flash Player is just a script that downloads and installs it from Adobe. The problem is that Adobe released a new version of Flash. The script downloads the new version, MD5 hashes the downloaded file, and compares it to the older version’s hash. The hashes being different prevents the installation from proceeding. (I’ve written about MD5 hashes before.)
Changes in the way the new version of Flash works mean that the Ubuntu developers can’t simply change the hash being checked for to match the new version. Until it gets sorted out (follow along in the bug report), you have two choices: install manually from Adobe, or use an unofficial updated package.
Either way, you should remove the broken package first. Paste this command into a terminal to uninstall it:
sudo apt-get remove --purge flashplugin-nonfree
Go to Adobe’s download page to manually install Flash. Or, follow the directions below for an easier way:
Run this in a terminal to download an unofficial package that has been fixed to work with the new version of Flash:
Run this command to install the package:
sudo dpkg -i flashplugin-nonfree_126.96.36.199ubuntu2_i386.deb
Restart Firefox and you should have the latest Flash Player working.