My wife strongly dislikes the new Firefox 3.6 tab behavior (where tabs opened from links appear immediately to the right of their parent tab, instead of at the extreme right of the tab bar).
I do like the new behavior, because by keeping related tabs closer together, it reduces the amount of time I have to spend interacting with the tab-bar scroll buttons (my least favorite UI element in all of Firefox).
She dislikes it for consistency reasons: when you open a new blank tab, it still appears at the far right. So now tabs can appear in two different places, depending on where you opened them. It violates the principle of consistency, which is generally considered one of the most important UI principles. This inconsistency hasn’t really bothered me personally. I’m not sure why; maybe it’s because opening a tab through a link, and opening a new blank tab, feel like different actions to me. There’s a difference in what I’m thinking about. But I can certainly understand how it feels like a consistency violation to other people.
My wife also doesn’t like that there’s no way to change Firefox back to the old behavior without going through about:config. (If you’re interested: type “about:config” in the location bar and hit enter, then do a search for a preference named browser.tabs.insertRelatedAfterCurrent and set it to True or False, as you like.)