Firefox-ui-tests has been moved and the Github repository is closed. Please use the version located in the following locations:
http://mxr.mozilla.org/mozilla-central/source/testing/firefox-ui/harness/
http://mxr.mozilla.org/mozilla-central/source/testing/firefox-ui/tests/
http://mxr.mozilla.org/mozilla-central/source/testing/puppeteer/firefox/
If you don't have a mozilla-central clone, or anything setup then follow those steps:
https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Build_Instructions/Simple_Firefox_build
Or, if you prefer to use Git then check the git-cinnabar documentation:
https://github.com/glandium/git-cinnabar/wiki/Mozilla:-A-git-workflow-for-Gecko-development
To see history, switch to the 'mozilla-central-deprecated' branch.