It’s the most annoying thing when every now and then, your flash version is outdated and it won’t load the pages you visit every day.
For me, this solved the problem:
First, install pepperflashplugin-nonfree. It’s avaialble from official repository on Ubuntu 14.04. But this only install the plugin for chrome.
Second, install the Fresh Player plugin from http://www.webupd8.org/2014/05/install-fresh-player-plugin-in-ubuntu.html
Restart Firefox and that’s it. Sadly, the plugin still consume a considerable amount of CPU resources. It feels faster than the official player from Adobe though.