To the best of my knowledge, there are exactly ZERO Iceweasel/Firefox add-ons that are supported for use in Tails (other than those that are already included). This means that any user who installs ANY add-on does so entirely at their own risk.

The general rule is not to make any changes whatsoever to the default configuration of Tails, with the only exceptions being: a) if the change is explicitly supported or, b) if you know exactly what you are doing and are prepared to incur any risks that may arise