Adding to the chorus! I've been a loyal Opera user for over 12 years and this is an essential feature for me. I always have many tabs open for various tasks that I need to revisit later on, and want to keep grouped together and out of the way while I do other things.
I would (very reulctantly) switch to another browser if this functionality was available elsewhere, but I can't find any Chrome or Firefox extensions that do manual tab stacking the way Opera did. There are many somewhat similar tab productivity extensions, yes, but none with manual tab staking that I can find.