Ok I determined that it was the flash player in the repo that was the problem, it's a bad version! I removed flash first of all:
sudo apt-get remove flashplugin-nonfree gnash
Then I downloaded a different flash player:
I unzipped the archive:
tar -zxvf install_flash_player_9_linux.tar.gz
Or simply right click the archive in the folder where you downloaded it and select "extract", then point it to the folder where you wish to save it.
Then I installed the new version of flash player by simply navigating to the correct folder, there are several versions in this archive so select the appropriate one for your OS.
I saved mine in the documents folder, 9r48 is for linux: /documents/fp9_archive/9r48/install_flash_player_9_linux
You'll be asked for the install path, for firefox it's:
Then I launched firefox and flash now works!