I had issues with Firefox not finding the plugins for flash. I had to setup the following environment variable for it to find it:

Check if it is already set:

echo $MOZ_PLUGIN_PATH

If it is not, set it:

export MOZ_PLUGIN_PATH=/home/user/.mozilla/plugins

Any path will work, I just used the .mozilla directory to keep things clean.

You should restart Firefox to see the plugins.

 

This was tested on Debian Wheezy.