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 and either approach may fix it once implemented / deployed by Sauce Labs.

Travis CI config to enable Firefox:

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

[noyainrain]

For status updates see noyainrain/micro#1 .