The current join behavior of ROBLOX on Firefox is that if you try to join a full server, ROBLOX will pend on the website without opening the client, allowing you to play in another server while you wait for the first server to become open. Chrome made an update that killed this behavior, forcing ROBLOX to change the process – Firefox is soon to follow. The new behavior opens up the client immediately, preventing you from playing in another server while you wait for the initial one to become open. A better solution exists though. Instead of closing out of the existing client when pending for a server, pend in the background of the existing client. Then and only then, should the client exit the current server and load the one they pended for. This is a much more graceful solution than forcing the player to stare at a blank screen for extended periods of time.
It’s more related to the fact that plugin support is being dropped (first from Chrome, now from Firefox), so we’re forced to switch to client-driven game join process, which means we need to launch the client before we get a server reservation. On Windows client implements single-instance mode (killing other client processes when a new one starts), which interferes with what you want.