Hi,

I have an old MacBook Pro model A1425 laptop that I'm wanting to install Trisquel Linux onto. I've got an external DVD burner with the .iso image DVD on it plugged into the laptop through the USB port on the Mac. I've tried to boot into the UEFI (BIOS in the Windows machine world) to see if I could launch into it and boot off the DVD but it would not do that. Instead it just keeps starting up Windows which is installed on this particular laptop. I've done this many times on Windows machines but this is the first time I've tried to install Linux onto a Mac. Any help would be greatly appreciated. Thank you

That's nice hardware even if it no longer on the market. It uses a far more advanced processor than what you can get in the stores these days. Since Trisquel is available only on that dusty, old x86 architecture its ISO images won't be able to boot on a more modern architecture like you have on the (now discontinued) MacBook Pro. Because it uses the PowerPC architecture you'll need a PPC aka PowerPC based distro or operating system. You can take a look at Debian, and modify that to remove as many of the nasty bits as possible, or else try OpenBSD. Either way, you will have to do a lot of tweaking of the appearance of the resulting desktop environment but you can take inspiration from any distro you wish.

See also: https://www.powerpc-notebook.org/en/

Edit: I misread the model name. The A1425 does use that dusty, old x86 architecture. You'll need rEFInd as mentioned below.

You have to create a partition the Mac EFI can see and boot the OS from.
From memory, I think it is hfs
Then use 'rEfind'
https://www.rodsbooks.com/refind/index.html

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Try this:
Burn MX to an USB pendrive, Power Off the Mac, hold the 'Option' key, then turn the Power on.
If it gives option to choose the USB, you are in luck - boot MX then install using 'Erase the entire disk'.

If you have an old MacBook Pro and want to get the most out of it, installing Linux could be a great way to go. It's a relatively simple process that will take some time to do but is worth it in the end. First, you must create a bootable USB drive with the Linux operating system on it. Once your device is ready, restart your MacBook Pro and press Option on startup. From here, select the USB drive from the list of available drives and continue with the installation process. With some patience and effort, you can soon enjoy the benefits of Linux on your trusty old Macbook Pro!