[Docs]: Try it out previews load the Add site screen in Firefox

[bgrgicak]

Docs previews load the Add site screen instead of the actual site in Firefox.

[bgrgicak]

After a bit more testing I found out this was caused by Enhanced Tracking Protection in Firefox.

[adamziel]

This issue got closed in favor of #1835. I understand you've meant to say there's no way to fix Docs site in Firefox private mode because:

• the enhanced tracking protection turns off a specific JavaScript feature necessary to run Playground
• there are no possible workarounds
• we can't detect the enhanced tracking protection is the source of the problem
• therefore, the only thing we really can do here is asking the user to switch to a different browser

I'll reopen this issue - would you mind documenting all that context so we know what to look for in Firefox changelogs and revisit when we're able to either provide a more informative error message or, ideally, get Playground to run?

Let's also aim to provide context in all the other issues before closing them, it will help us revisit these decisions in the future.