Unfortunately TenFourFox dose not support plugins such as flash due to it being out of date and a security risk, If you would like to use flash you can use TenFourFox 17 or older or just use Safari but at your own risk. As far as flash goes some web producers seem up to converting things to HTML 5 but their are still those who are staying with flash.
The last version of TenFourFox that officially allowed plugins
without restriction is 5.0.
Tne last version of TenFourFox that unofficially allowed them
(with a change to about:config) is 17.0.11. If you set
tenfourfox.plugins.enabled to true then
they will run in that version. We don't support, won't offer
assistance and won't accept bug reports for that version, it does
have known security holes, and you use it at your own risk. It is
downloadable from the file archive on SourceForge: https://sourceforge.net/projects/tenfourfox/files/archive/