Well, for anyone who doesn't see the point here, the default of Firefox should be nice and have taste. When someone wants Firefox to look like a Chrome replica, well, then you can go for having a custom theme, too. If someone doesn't get, what is the...
I can just second the idea of making it configurable, but there must be a consolidation too in the moment it's configurable - Otherwise you start accumulating lots of settings around design details that an average user expects to come out of the box ...
Firefox 157's rounded tabs are a regression, not progress. This is a direct copy of Chrome's aesthetic — and a bad one. If I wanted Chrome's look, I'd use Chrome. The rounded tab shape has no visual connection to the content area below it. It floats....