Unfortunately, Adobe has created a situation that breaks Flash plugins for Firefox in Ubuntu. This tutorial will help you fix those problems.

Step #1: Remove Gnash
Gnash is an Open Source alternative to Flash and is often installed in an attempt to get things working when Flash fails. Well, this is not a solution so you need to remove Gnash. To verify if Gnash is installed just take a look at the pluginreg.dat file listed below.

Contents of the fiel .mozilla/Firefox/pluginreg.dat

Shockwave Flash 8.0 r99. Gnash 0.8.1, the GNU Flash Player. Copyright © 2006 <a href="http://www.fsf.org">Free Software Foundation</a>, Inc. <br> Gnash comes with NO WARRANTY, to the extent permitted by law. You may redistribute copies of Gnash under the terms of the <a href="http://www.gnu.org/licenses/gpl.html">GNU General Public License</a>. For more information about Gnash, see <a href="http://www.gnu.org/software/gnash/"> http://www.gnu.org/software/gnash</a>. Compatible Shockwave Flash 8.0 r99.:$
Shockwave Flash:$
1
0:application/x-shockwave-flash:Shockwave Flash:swf:$

You must be root to use apt-get so become root by doing:

sudo apt-get remove mozilla-plugin-gnash sun-java6-bin gnash
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
libboost-thread1.34.1 libboost-date-time1.34.1 gnash-common
Use 'apt-get autoremove' to remove them.
The following packages will be REMOVED:
gnash mozilla-plugin-gnash sun-java6-bin
0 upgraded, 0 newly installed, 3 to remove and 11 not upgraded.
6 not fully installed or removed.
Need to get 0B of archives.
After unpacking 1069kB disk space will be freed.
Do you want to continue [Y/n]? y
dpkg: error processing sun-java6-bin (--remove):
Package is in a very bad inconsistent state - you should
reinstall it before attempting a removal.
(Reading database ...
dpkg: serious warning: files list file for package `sun-java6-bin' missing, assuming package has no files currently installed.
94840 files and directories currently installed.)
Removing mozilla-plugin-gnash ...
Removing gnash ...
Errors were encountered while processing:
sun-java6-bin

You may see some errors, however, I found I could not get it to be removed unless I used this command.

Step #2: Install the Correct Flash Version

The key now is to install the correct version.

This is related to a bug #173890 for Ubuntu but there is a fix in the comment at #212. Run these two commands.

wget http://kubuntu.org/~jriddell/tmp/flashplugin-nonfree_9.0.48.0.2+really0ubuntu12.2_i386.deb

sudo dpkg --install flashplugin-nonfree_9.0.48.0.2+really0ubuntu12.2_i386.deb

Restart your Firefox browser and you will see that it is now corrected.