What could be clearing Firefox Cache?

I'm accessing a remote desktop Windows server. I have installed Firefox Portable. When I login to the site I reach two-factor authentication. I've authenticated it manually and ticked the box 'Don't ask for codes on this device'.

Each time I relaunch Firefox it works without having to reauthenticate, but each time I close the remote connection and reconnect, the Firefox cache appears to have cleared and I have to go through 2 factor authentication again.

Have you any idea why the cache would be clearing? I sometimes run CCleaner manually but even when I do that, I can still login without 2-factor, so I don't think it is ccleaner causing the problem. I think it is something to do with reconnecting to the server.

I use an IP address to login and that doesn't change. I've checked whatsmyip.org and it matches what I login with.