Mozbase has moved! The github repo is closed. Please use the version located in mozilla-central/testing/mozbase. If you don't have a mozilla-central clone, you can obtain one by running:
$ hg clone https://hg.mozilla.org/mozilla-central
Or, if you prefer Git:
$ git clone https://git.mozilla.org/integration/gecko-dev.git
To see history, switch to the 'old-master' branch.