Okay that helps. Based on the dependency chain it looks like flashrom and libftdi1 are using the new library with the compat layer.(I'm slightly fuzzy but hope this helps.) The bit picture is that there are two versions of libusb. There is an old one, libusb-0.9?, and a newer one, libusb-1.x. There is then a compat package that provides the libusb0 API while using libusb-1. pkgsrc has gradually been converting over. See /mk/libusb.buildlink3.mk and perhaps libftdi1's way of including libusb is not really right, as it seems to use the wrapper mk and also directly.
Okay that might have helped. I also build apcupsd which just includes: mk/libusb.buildlink3.mkThat package would have been built BEFORE flashrom so if LIBUSB_TYPE defaults to 'native' that would have installed the OLDER libusb. Looks like I need to set LIBUSB_TYPE=compat before building apcupsd if I want flashrom support as well.
I'll try that out on the next build (which I've just kicked off). That should also test the thunderbird/firefox68 change. :)
Thanks for the hint. Mike