Let's say I'm browsing from Ubuntu Virtual Machine using Firefox.
IF the website I am visiting can see the MAC Address, which MAC address will it see?
Is MAC Address of Ubuntu VM or MAC Address of the host Operating System?
The website you are visiting can't see the MAC address of your computer, unless it is at the same LAN.
For a computer on the same LAN, it depends on your VM settings. Bridged access will see the MAC of the guest, while NAT access will see the MAC of the host.