booting USB installers on Librems with PureBoot firmware should be as easy as selecting options/USB boot/[first boot option]
when installing, it’s important to use a dedicated /boot partition – which IIRC Ubuntu doesn’t do, not sure about Mint. After installation, sign the new /boot files, and then set the default OS boot option (again, selecting the first/top one).
I’ve installed / tested PureOS, Qubes, and a few others this way
Depends what you mean (regarding “doesn’t do”).
Looking at a selection of Liveboot USB flash drives for Ubuntu 16.04, 18.04.1, 19.04 and Mint 19.1 they all have two partitions, with the second partition being of type EFI (and containing a FAT file system).
Looking at an actual installed Ubuntu 19.04 system, partition 1 is type EFI (FAT file system mounted on /boot/efi) and partition 2 is your regular ext4 on /
Looking at an actual installed Ubuntu 18.04 system, there is only one partition (your regular ext4 on /) but it could also depend on the upgrade history, as this system has been upgraded from earlier versions of Ubuntu, and it may depend on the age of the hardware.
Sorry I don’t have an actual installed Mint 19.1 system to check on.
installing any recent Ubuntu desktop version on a Librem with either our standard SeaBIOS or PureBoot firmware, with the ‘erase and install’ option where the installer sets up the partition layout, will result in a single ext4 partition - which is not ideal for PureBoot. One has to manually set up two ext partitions – one for /boot, one for / – for PureBoot to work optimally
yes MrChromebox means that Librems’ PureBoot is non-uefi thus it needs a separate /boot just like a proprietary modern uefi-bios requires a distinct /efi partition.
god it’s so effing confusing to live in a world where you always have to pay attention to these differences. ugh !
Yes. That’s why Purism can say that PureOS works with Purism hardware running either of the two firmware options (SeaBIOS or PureBoot) - and anything else you might have to do some of the work yourself to iron out the differences.
Some differences are good differences. Some differences are bad differences. Some are innovation. Some are just to be difficult.
With the emphasis on “ext4”, since, as noted above, you do get two partitions - but the “boot” partition may be of the wrong partition type (can easily change later on) and has the wrong type of file system (can easily change later on).
Is this the sort of “adventure” that I want to spend my time on? No, not really. 
Shouldn’t this be detailed in a pinned post under the “Other OS” category instead of casually dropped in one of the hundreds of threads? That seems kind of important …
What is the recommended partitioning for installing a new operating system on a librem with pureboot?
I am considering installing Fedora 31 from a USB stick, the process would be similar to the one described in this thread, I will be give the choice to let the system do it or manually do the partitioning.
Right now I have PureOS and 2 partitions:
/dev/nvme0n1p1 1.1G
/dev/nvme0n1p2 475.8G
Thanks!