If automatic updates for Shockwave Flash (Firefox) are enabled then optional offers will NOT be installed.
Also, The simplest and most straight forward alternative is to uncheck the checkbox. It’s an opt out situation, not opt in.
To clarify, there are two types of auto-updates:
Notification auto-updates: a notification displays indicating a Flash Player update is available. This type of update will provide the option third party offers.
Background (silent) auto-update: Flash Player updates silently in the background without any user interaction. This type of update will not include any third party offers.