I haven't read through all of the feedback above, but I can keep my own statement very brief.
I find this way of simply taking away user interfaces that the user has come to love and familiar with and replacing them with a horribly ugly "new" one, as in the case of One, to be disrespectful and arrogant. In this way, you can very purposefully book market shares in the dustbin.
Just offer a "classic theme" option and everyone would be helped. How did the One Theme actually make it through quality control?!
I'm now switching to another browser and will see in 2 months whether you still don't care about your customers/users. In that case, you only have to uninstall your software.
Unfortunately, that is a pity!
Please reconsider your product strategy.
Best regards
a frustrated and disappointed user