Found a temporary workaround on 52.6.0esr-2pureos1:

Possibly related: I'm having a similar issue when trying to run several installers or standalone EXEs.
Doing "wine foo.exe" (where foo.exe is known to be fully compatible with wine) yields:

it looks like wine32 is missing, you should install it.
as root, please execute "apt-get install wine32"

A lot of Windows applications are still distributed as 32-bit binaries to avoid compatibility issues, so this kind of makes sense. But wine32 is not in the main PureOS repo because (IIRC) PureOS is all 64-bit code. So i386 isn't an enabled architecture, nor does the repo distribute 32-bit software.

@francois Yup, it absolutely should. Edited.

That seems to have worked. I ran into some additional issues with the storage pool, but that was mostly from me trying different things. Spun up a fresh user, added it to the kvm group, and successfully created & started a VM. Troubleshot the other issues on the main account, and successfully created & started a VM.

This issue persists on purebrowser 52.6.0esr-2pureos1.

@tharvill That was the case. Did:

Possibly related: I had a couple of Firefox extensions installed prior to upgrade that I can no longer install via addons.mozilla.org - I was previously able to, but now I get an error saying I need to install Firefox in order to install.

My system after the most recent updates is running the following versions:

Also experiencing this issue - can't start any VM from a known good ISO. Running PureOS OEM image, gnome-boxes is 3.26.2 - issue persists across multiple DEs and reboots.