The stumbleupon extension was removed from the archives a few days ago [0]. The app-install-data-ubuntu package has not been updated since then, which is why it is still being displayed in software-center.
[0] https://launchpad.net/ubuntu/lucid/+source/stumbleupon/3.3.8-3build1 ** Package changed: software-center (Ubuntu) => app-install-data-ubuntu (Ubuntu) -- no install button displayed for the stumbleupon firefox extension. https://bugs.launchpad.net/bugs/572859 You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
