Cannot Install or Remove Flash Plugin
Sometimes, Firefox will show up as a broken dependency in Synaptic Package Manager due to adobe-flashplugin missing. This usually occurs when Ubuntu Restricted Extras is not installed. Because of the broken dependency, one will not be able to install URE or make any modifications to Firefox.
In order to fix the broken depency, execute the following:
find /var/lib/dpkg/alternatives/*flash* -type f -size 0 | xargs sudo rm