fix: handle Firefox location on Windows

Currently the task assumes Firefox is located under process.env.PROGRAMFILES
directory. This is not true, as Firefox, being 32-bit on Windows, is located
in process.env["PROGRAMFILES(X86)"]. This commit adds a fallback to the second path,
being forward-compatible in case 64-bit Firefox gets released for Windows.

Analogous PR for Chrome: https://github.com/karma-runner/karma-chrome-launcher/pull/6