Provide better testing for Tor Browser not breaking important websites (Twitter/Github etc.)
We recently had to deal with Twitter being hardly usable for Tor Browser users due to a bug in our code (see #16450 (moved) and #21555 (moved)). We might want to have some tests informing us as early as possible if such a breakage happens. Selenium might be a good tool for writing such tests, maybe Marionette as well.
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