But how so?
Look at how Opera UI was before Chrome and after Chrome. (10.10 -> Chrome launch -> 10.50 all the way up to 12.15) it doesn’t look more like Chrome. I think the only difference is the way there’s a menu button at the top left corner (making Opera tabs which were already above the address bar to merge with the title bar). And you could aways use the address field for searching and remove the dedicated search field.
I don’t have the source but Firefox 4 UI is said to have been planned since before Chrome existed also (people actually said Fx copied Opera at the time and others said Opera copied Fx because of the similarity at the time, the releases were so close it caused confusion!)…
I don’t see how they became more minimalistic because of Chrome… The timeline doesn’t match and Opera stayed with its status bar (removable since a long time ago) for example… Now, IE9 is really very suspicious on adopting minimalism only after Chrome because we don’t have access to the internal UI designs they made between 2008 and 2011.