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I recently got a Raspberry Pi 3 Model B+. Apparently this is quite new as of asking this question? On my previous Pi a Raspberry Pi 3 Model B I was running a version of Ubuntu 16.04 LTS 'classic' that I downloaded and flashed onto an SD card with etcher (from this url https://wiki.ubuntu.com/ARM/RaspberryPi) without a problem.
I got the new Pi in the mail stuck that very same SD card in and tried to boot it up and I get stuck on the rainbow screen. It won't boot up. I put the SD card back in the previous model and it boots up just fine.
Some people have mentioned that the firmware is different and needs to be updated? But all of these mentions are referring to Raspbian not Ubuntu.
Is there a way (or need) to update the raspberry pi firmware from Ubuntu?
Any other reason why my Pi might be stuck on this rainbow screen and refusing to boot?