@up
I have 40-50 tabs open all the time and that's why I need tab stacking so much. It requires 2-2,5GB of RAM and I'm absolutely ok with it. The same set of tabs requires only 1,5GB in Firefox but it's really not a problem with modern computers. RAM is cheap.
The problem with the old Opera is that it gets incompatible with everything around, flash being the top issue. Every now and then something gets crazy and something like that happened to youtube and embedded videos after the latest flash update. At least on my machine. When I try to watch any youtube videos, Opera's use of RAM spikes to over 14GB. I use the "click to activate the thingy" function (don't remember how it was called in english and don't want to come up with an even sillier translation) function so it doesn't bother me in normal activity, all the same, youtube has just joined the ever-growing list of sites incompatible with Opera 12.
I'm extremely unwilling to switch browsers but it seems that opera's devs, teamed up with the universe, are hell-bent on forcing me to do it.