First click the link provided to get 'personas plus' for firefox (it's a free add-on).
Once you have installed it and restarted:
[1] Click the firefox 'Help' tab,
[2] Click on 'Troubleshooting information'
[3] Click 'Open containing folder'
[4] Open 'personas' folder, then the 'cache' folder, then a numbered folder and there you will find the two images from your current persona.
https://addons.mozilla.org/en-US/firefox/addon/10900/