I find that Chrome is actually worse with lots of tabs, even though it doesn't lose as much speed when you have lots of tabs open. Because Chrome keeps tabs in a separate process, I can quite happily view two or three tabs that I have open and use constantly without any loss of speed, even if I have another 12 tabs open in the background that I want to check at a later date. However, when I do want to check them, say, every 20-30 minutes or so (e.g. Twitter, Facebook or Tumblr), it takes 3 or 4 seconds for Chrome to load that tab, because it has to load all the pictures and stuff from its cache, rather than from RAM. Presumably, Chrome decided to put the processes to sleep and store the data in the cache until I needed them later, which frees up memory and makes the rest of my browsing less clunky, but it does make it rather frustrating, since I check twitter frequently, spend less than 30 seconds reading them, but want the new tweets right now, rather than in 3 or 4 seconds. It would be a lot quicker if it actually loaded the page from the web instead of the cache.
The way Firefox does things (in 3.6 - haven't spent much time with 4 yet) makes general browsing a lot slower, because it doesn't put stuff to sleep when I'm not using it, so all the open tabs are still taking up precious RAM and processor time, but it makes it a lot more functional for checking twitter, fb, tumblr, etc the way I do.
Of course, that's nothing a bit of extra RAM wouldn't fix. And I've generally changed my surfing habits to compensate (closing the twitter tab and opening it when I want to check it in Chrome, so that it doesn't have to access its cache before showing me the goods).
I don't know how Opera does it. And given that most of the people who use Opera here seem to have a lot more RAM than me (and generally better computers), I doubt they'd face similar problems to me anyway.
