karma: Filter extra Firefox builds on Linux
karma: Filter extra Firefox builds on Linux

Somehow karma-detect-browsers detects Firefox, FirefoxNightly, and
FirefoxAurora from the presence of a single `firefox` binary.

mbland/custom-links,mbland/url-pointers,mbland/custom-links,mbland/custom-links,mbland/url-pointers,mbland/url-pointers