I hope you inform them before installing it. If someone installed something that's tampering with my browsing experience without telling me I would be pretty mad.
After I wrote the above, I double checked myself and, apparently, I am using Mozilla's definition. However, it seems Apple also differentiates among the terms add-on (the super-set), and extensions/ plug-ins (subsets).
If someone makes my life immeasurably better without telling me, I'm thankful.