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?
Its a new feature in 12.10 called Web Apps.
Here is the definition of web apps provided in Ubuntu site.
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.