Mysterious Firefox addon installed

I've got two machines,both 10.04, one straight ubuntu, the other xubuntu. I can't remember when I first noticed this because things have been pretty hectic, but there's a Firefox addon called ubuntu firefox modifications 0.9rc2, subtitled ubuntu firefox pack that's listed in the info box that pops up when I click manage add-ons. I never put that on either machine, and when I go to "search all addons" to see what it is by typing that name in, the name that comes up is NZBGet UI. The description there tells me nothing about why I would want it, but it's geeky enough for me not to really know what it is anyway. Googling it gives me "Did you mean NZBGet GUI" The entire page below has no GUI entry. No one else has access to these machines. The firefox info pages seem to be a mystery in themselves, so I decided to ask here first: does anybody know how this addon got on these machines and/or what I should do about it? Thanks In advance.