Firefox in Ubuntu 12.10 pops up messages such as (similar messages for Reddit, Facebook, etc.):
"Would you like to install Twitter for extra features and quicker access?"
What do they mean? What exactly gets installed and where?
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Its a new feature in 12.10 called Web Apps.
Here is the definition of web apps provided in Ubuntu site.
Ubuntu Web Apps enable developers to create web applications that run in web browsers, but act as if they are native applications. They provide close integration to the Unity shell for functions such as launch, notifications and controls.
If you agreed to install the FacebookMessenger, you would see that a package named unity-webapps-facebookmessenger was installed in /usr/share/unity-webapps/userscripts/unity-webapps-facebookmessenger. Other websites do the same. Search for the corresponding package in Software Center.
Webapps integrate into Unity with a launcher icon and in the messaging menu.