Every time my Windows-on-virtual-box gets a new Opera, I just rename it to Opera(version) at save (by default it wants to be called "Opera"). So the desktops there are riddled with Operas from 9.28 and up : )
For Firefox I found the whole setting profiles thing was not only a crapload of work, but was so easily lost when updates came. Luckily, I can keep K-Meleon for Windows as a wholly separate browser. It's updated but still (so far, fingers stay crossed) uses the older Gecko rendering engine used by FF2... so it's great for testing -moz-inline-block vs -moz-inline-box and some of the bigger bugs it had.
My firefox on Linux is always several versions behind bleeding edge, mostly because I go for long times before allowing the update manager to update it. Mostly because I don't want to have to actually deal with the new "features" Mozilla comes up with... meanwhile on the virtual boxes the firefoxes get regularly updated as often as possible. I'll stop if an important plugin like WebVisum stops working though.
Chrome I haven't figured out. I regularly get "updates" without version numbers and have no clue what's going on there... but I suspect Chrome on Linux lags Chrome on Windows so I consider my Chrome "old" and the virtual box one "new" for versions.
Safari so far since I have a virtual box for IE6 and one for IE7, there's Safari 3 on one box and Safari 4 on another box. A machine at work has IE8 so there's Safari 5, but that could be another virtual box (not sure how many my computer can handle running at once before it starts dividing by zero and 'splOding! or something though...).