After many attempts, I have a fix for this. The problem lies with Firefox version 39 and the Flash plugin. Either the Adobe or the Pepper flash.
What I have done is totally delete the flash plugins, install the Pepper flash, and download Firefox 39 from Mozilla, and overwrite Firefox 39, which came from the Ubuntu repositories.
The very first thing is to delete all Adobe flash installations
dpkg-query -l | egrep 'flash|fresh'
Now delete these with the purge option, using the file names returned from the dpkg-query command:
dpkg --purge <file_names>
Now you can install the pepperflash from webupd8:
sudo apt-get install pepperflashplugin-nonfree
sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install freshplayerplugin
Now you will need to download the latest version of Firefox from Mozilla, as the version from Ubuntu seems to have issues. First step is to backup, for just in case something breaks:
sudo tar -cvPpf ~/Firefox-backup.tar /usr/lib/firefox*
sudo wget -P /usr/lib/firefox https://download-installer.cdn.mozilla.net/pub/firefox/releases/39.0/linux-x86_64/en-US/firefox-39.0.tar.bz2
sudo tar -C /usr/lib/ -jxvf /usr/lib/firefox/firefox-39.0.tar.bz2
Now you should be able to start Firefox and watch video without the constant crashes.