"Can I test trisquel 7 64bit on my computer on an usb stick?"
Sure.
"If trisquel does not contain non free software and no mainboard works
without non free software, then how can it work?"
The mainboard will contain proprietary software in the form of the BIOS or
UEFI (depending on what kind of board you have) but that doesn't mean
Trisquel itself needs to contain non-free software in order to be bootable on
those boards. :) They're still capable of booting 100% free GNU/Linux
systems. It's just that, since the software is proprietary, it's not doing
you have favors because it denies you -- the supposed board "owner" -- of the
needed freedoms.
