In this post on an Ubuntu forum thread someone poked around the package contents to see what the addon's source actually does when enabled. It lists the following functions:
- Enables usage of Ubuntu plugin finder service.
- Allows localization of extension descriptions.
- Sets several browser defaults, like printing, look-and-feel, disables checking for default browser, locale matching with OS, etc.
- Adds Ask.com search plugin.
One undesirable side effect is that it seems to interact oddly with a number of other extensions1 such that you can not add their toolbar buttons to any toolbars, even on the default theme2. Disabling the Ubuntu extension re-enables adding these buttons to toolbars.
Footnotes:
1. most notably FlashBlock and TabMixPlus in my case
2. some people suggest the problem is related to theme choice, I have found this not to be the case