You can't do this with Thunderbird. Assuming you're using Windows, you could use a separate user account and configure the other email account there, which would somewhat limit its access. This is sufficient in most contexts where there is implicit users trust (for example, a family computer). However it's rather trivial to install a hardware or software keylogger to gain access to it. If you haven't done this yet, I'd suggest you consider your [https://en.wikipedia.org/wiki/Threat_model threat model] and why you want to separate those accounts while keeping them on the same computer.