That's not always true, and sadly aesthetics can be a killer in retaining customers. Mozilla knows this, and as a result the next iteration of the infamous Firefox will go to great lengths to blend in. Firefox 3 will rely more upon the theming of the host OS, with the aim of making it appear seamless and polished with the interface around it. While today Firefox can get in well with many themes, it is usually pretty obvious that there is a lot of room for improvement.
I hope they do not sacrifice quality and stability over aesthetics, but some of the integrations they are describing sound well worth it:
"I personally think Firefox has in the past established its identity through interactions as opposed to the visual design of the interface itself," he said, citing for example people's recognition of the tabbed browser windows in Firefox 1.0. And users similarly might identify in Firefox 3 with a feature that lets them navigate to a Web page by typing some part of its name in the location bar, with Firefox suggesting full links based on bookmarks and previous pages visited.
I'm usually not one to judge based on the appearance of software, but if upgrading the aesthetics can make the overall browsing experience feel more natural, I'm all for it.