Funny, that's why you couldn't pay me to use Thunderbird over GMail. Incidentally, if you are interested, there are ways to manage multiple accounts, though they're a bit clumsy. Basically, you can have GMail download your other email accounts through POP (or have them forward to your gmail account). Then in gmail you can create a rule that automatically labels email from that account appropriately and create another inbox to handle just that mail. Finally, it lets you set up an alternate sending address, and you can set it to always reply with the same email that the original was sent to.

I use Gmail to manage four separate accounts.

You can set filters that label each message based on sender, or any search really. For example to:[email protected] gets labeled and color coded.

You can set up multiple inboxes for your accounts.

Also it's easy to switch between accounts when replying, to reply from a different address for example.

Gmail is just too good for me not to use it, but use whatever works for you.