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
Can we use `#{Travis::Build::HOME_DIR}` instead?  Sure, it's the same as `$HOME` for now :wink: ...