Similar to how Brave implements this feature. It would be beneficial for users and will provide enhanced privacy, should a user require it.
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Well, there is one big problem in brave's implementation of tor in private windows. Tor works by blending you with the crowd, thus making you anonymous. For this, your browser fingerprint should be the same as others. That's why it is recommended to never install extension in tor as they change the fingerprint.
As brave is chromium based, it gives a completely different fingerprint than the official tor browser and so the websites can easily fingerprint you.
As tor browser is firefox-based, firefox could give almost same fingerprint but I still think it's not worth implementing tor mode in firefox. Not everyone needs it and it would make firefox harder to maintain for the developers.