e2e Tests: Run against Firefox

It'd be great if we could run our e2e tests not only against Chrome, but also Firefox. We've recently experienced an [issue](https://github.com/Automattic/jetpack/issues/17850) that broke Jetpack blocks when used with Gutenberg 9.4.1 in Firefox, but had no impact on Chrome. It's quite likely that our e2e tests would've discovered if they'd been run on FF, once they'd picked up GB 9.4.0 or 9.4.1.

According to some sources I found, it seems like puppeteer started [supporting](https://github.com/puppeteer/puppeteer#q-what-is-the-status-of-cross-browser-support) Firefox "properly" (without the legacy `puppeteer-firefox` package?) in [v3](https://github.com/puppeteer/puppeteer/releases/tag/v3.0.0), which was released earlier this year. We're currently still on [v2.1.1](https://github.com/Automattic/jetpack/blob/7834c62b9c29a0133a5b8433c0f432b8b9ba6ebc/package.json#L247). There's been a renovate [PR](https://github.com/Automattic/jetpack/pull/16585) to upgrade to the current version (5.4.0!) for a while -- it might make sense to land that before attempting to add coverage for Firefox.

cc/ @brbrr @bsessions85 @jeherve