Smoke feature needs @firefox tag to run on Wikimedia Jenkins

Fixes jobs:

-
https://integration.wikimedia.org/ci/view/BrowserTests/job/browsertests-
CirrusSearch-en.wikipedia.beta.wmflabs.org-linux-firefox/
-
https://integration.wikimedia.org/ci/view/BrowserTests/job/browsertests-
CirrusSearch-test2.wikipedia.org-linux-firefox/

Paired with: Antoine Musso
Bug: 58040
Change-Id: I0137e3c2cebebd6f027f0201b3c321c92804d0b5