I don't know what it is, but I agree. There is something subtly and subconsciously going on that nudges me away from FF too. I think it's the loose feeling UI, the default smiley face icon of the chat that I don't know anyone uses, the paper airplane as the share icon. It's that there is still not unified search/address bar even if one would prefer it without running a poorly executed extension. It's that the tabs feel so childish with the far too rounded corners and loose padding and it's even the few pixels more of padding between the address bar and the tabs boundary. It's the small font of the address bar, it's the disproportionately large back button, it's the apparent lack of design and style requirements for extension icons that make them look blurry and generally shitty and bolted on in the UI, etc.

I know that all that maybe sounds rather petty and maybe there's something wrong with me and it's my biases for some reason, but it just all adds up to a fuzzy notion of childishness and less than "down to business" feel. I say that as someone that used to exclusively use FF and shunned Chrome for all the various reasons mentioned by others.

I don't get how he thinks it's ugly or different.

Not only is it very similar but, unlike Chrome or Safari, every aspect of it is customizable so you can make it look however you want. Out of the box you can rearrange the the UI elements to match Chrome or Safari.

It's quite different, and because of XUL it also falls in the uncanny valley of looking like native widgets but not behaving exactly like them.

I'd say FF Developer Edition has the best UI out of all of them.

Nothing fits better next to a terminal window or a code editor.