Enable Firefox in test-ui stage

[noyainrain]

Making use of advanced user interactions fails in Firefox with remote error: UnsupportedOperationError: mouseMoveTo. This problem is addressed both for the Selenium Node.js client and the Selenium server.

There is an experimental branch using a pre-release version of the client library.

On the server side, the problem should be fixed in Selenium 3.5+, but Sauce does not yet support it in combination with Firefox (test starts, then times out).

Travis CI config to enable Firefox:

jobs:
    include:
        - <<: *test-ui
          env: BROWSER=firefox PLATFORM="Windows 10"

[noyainrain]

To expand menu via hover (instead of click):

await browser.actions().mouseMove(menu).perform();

[noyainrain]

Since #14 menus also expand on focus, which works in Firefox, so for now we have no need for advanced user interactions any more.