Install Firefox add-on under `$HOME` without sudo

And modify `$PATH` so that `firefox` can be found

We can't use `ENV['PATH']` or `ENV['HOME']` to set values,
since the build script is compiled by the build API, which
has different values.

You can see the result here: https://staging.travis-ci.org/BanzaiMan/travis_staging_test/builds/392848#L52
I _think_ the escape comes from a time when I was trying to figure out why things were failing (because some variables are evaluated on the API machine, not the worker). I'm down with using `HOME_DIR`.