Comment 14 for bug 139138

Sudhanshu-
We might be taking about different problems, the bug I ran into was that flashplugin-nonfree wasn't installed at all. Then, when firefox asked to install it, the installer bombed out because of a bug (I think because when I installed, I didn't have an internet connection, so there were no repositories configured that had flashplugin-nonfree (many were commented out)). However, firefox misread the installer return value and said the installation was successful when it actually failed.

Your method will work fine, but there might be problems down the line if another program tries to reinstall flashplugin-nonfree. I don't know if the install script is smart enough to remove the old libraries before installing. If so, you might run into conflicts.