Have you tried Prism (for firefox)? It creates an app out of any url. Since each app has it's own set of preferences and cache, you can make an app for each of your gmail accounts. That's what I do. Works like a champ.

I have used Google Chrome's Stealth mode to truly open gmail accounts separately.

I share your pain.
I have taken to having my 3 primary accounts simultaneously logged in in google chrome, with a notifier plugin that allows me to switch directly to any of indexer accounts in a new tab - which takes away my main frustration that switching accounts using the google drop down replaces the content if the current tab. For the fourth account, I have that open in opera.
Not perfect by any stretch of the imagination, but still infinitely better than trying to manage multiple mail accounts in something like Outlook!