Apparently it's a little cloudy how to get the latest plugins for portable firefox, where to find the dll's after updating flash.
1. Close your portable firefox.
2. Open firefox on the local PC.
3. In URL write: about:config
4. In search: plugin.expose_full_path
5. Set it: true
6. In URL write: about:plugins
And on this page it will now show the directory locations on local PC for every addon installed, so you can copy it to your portable plugin directory
Last: go back to about:config and plugin.expose_full_path and set it false again