I think I found 2 useful links:
The first is a Firefox and Thunderbird extension that can protect your profile with a password.I haven't tested it for portability and I think the protection isn't very strong but it may suffice for average stuff.
The second is a Thunderbird extension that lets you brows within Thunderbird. I haven't tested this one either so I don't know what browser it uses.
Just wanted to share them with the community.