Who are your users?
If you simply copy your competitor, then you lose.
Why should I use Firefox when I can get Chrome for the same price - since Australis, they now look almost identical and Chrome is faster.
Firefox' USP was the customisation. I could install many extensions/addons that customised the way it looked and worked.
Not just colours and textures, but layout and some UI behaviour.
Yes, that all came at a cost but the cost was shared between Mozilla and the extension authors.
Take that away, and what is left to recommend Firefox over Chrome, Edge, Opera or Safari?
If they all look and behave the same, then I should pick the cheapest or the fastest. Firefox isn't either of those.