I would keep the last Facebook-free Opera as long as possible. As said before, it depends on how Opera would change.
I would also need a hardware update to run FF or Chrome without running out of memory.
I actually switched from Opera to Firefox (Pale Moon) because Opera was starting to use more memory!
I have 100+ tabs open in Pale Moon right now and it's only using 380MB of RAM. This is because I have a modified version of the BarTab extension running that only loads the tabs I've viewed in the last 10 minutes.
If Opera had a tabs-on-demand feature like Firefox, I would switch back to Opera in an instant! But until then, I have rekindled my interest in Firefox.
For me, Firefox is the one that comes closest in functionality (and trust), although it is slower. And I use Iron for Youtube, because it handles flash better.
I will have a second look at Maxthon as my third option.
Chrome is for 18+ stuff only!
Let's see Google making a database of that browsing habits!
Firefox has a private browsing mode as well.
As for Iron, I'd recommend Chromium Portable instead as it has a smaller package and is pretty much the same. Iron's "enhancements" are too minimal to be considered a fork of Chromium.
Maxthon gets cool points from me for trying to innovate their UI from the stock Chromium interface. I would probably tell them to get rid of the Trident engine and just focus on Webkit/Chromium.
The one thing I don't like about all Chromium browsers is that it uses a ton of memory! If you use Adblock Plus in Chromium, it will take up 80MB of RAM alone! View your Chrome task manager and see how much memory it takes. Of course, you could state that Chromium is more secure because it uses a different process for each tab / plugin, but I don't like that.