A quick tip. If you're running karma tests via Firefox on Travis CI by default the Firefox version used is quite old (31 at time of writing).
To always use the latest (current) version you can add this snippet to your
addons: firefox: latest
This will keep your firefox up-to-date as new versions are released. Two other special release aliases are available:
latest-esr- The latest extended support release.
latest-beta- The latest beta version.
You can of course specify a specific version like so:
addons: firefox: "44.0"
For for info see the Travis docs on setting firefox versions here.