I've been using Firefox-1.5.x again on wine (using the winehq packages for Ubuntu) on my dual 650MHz system as my main browser again for the last few months. It's working quite well... except that javascript-heavy pages/apps like gmail are a tad sluggish at accepting keyboard input. For some reason, yahoo.com's main page is also sluggish; they must do something fancy. Both are fine with native firefox on this system.
I'm sure this won't affect anyone with a modern system, but it is kind of sad that my trusty old dual CPU system isn't up to daily tasks anymore. Sniff... ah, well, I see Fry's has amd64 4000 motherboard/cpu combos for $189, guess it's time to upgrade.
At some point it'd be nice to look into why Firefox's javascript is slow under Wine, though. - Dan