Support Multiple Browsers

[mattthaber]

Firefox is supported in puppeteer now, would be nice to support multiple browsers that could be configured via ENV variables potentially?

[noahnu]

Rather than an env var, we could pass the browser as a config option. There's the question of whether we should have 2 lambda functions, one per browser, to keep the size of the docker image down. In this case, we pass a map of browser/env to lambda functions so a single yarn sanity-runner-client execution could use different lambda functions per test.