Fixed this issue.
Also, thumbs were displaying all the way across my modal window off the screen for me on FF (as it looks like it is for OP.)
To fix, add the following css to your theme's "styles.css" or the plugin's "prettyPhoto.css" located at YOURSITE.COM/wp-admin/plugin-editor.php?file=wp-easy-gallery/css/prettyPhoto.css
after the last line.
Without this css fix looks like this...
With this css fix looks like this...
Plug in dev.. this probably needs to be added to your css, not sure why it wasn't working before but I came here looking for a solution to this very problem.