I have just got an answer, @2020matters; from Sébastien.
He confirmed that they are aware of some users' difficulties with Safari. I pointed out that it remains the browser of choice for many Mac users.
I am now able consistently to use FireFox for download purchases, though.
One worry, I'd have thought, is that - dedicated and extremely knowledgeable though the Mozilla developers are - if Qobuz fails on Safari, perhaps because of some minor incompatibility, a version of FireFox could perhaps be released (given the frequency with which it is updated) which also prevents it (FireFox) from working with Qobuz.
I would still be uncomfortable compromising my otherwise 'clean' machine with all the apparently irremovable modifications and extras which Chrome installs.
As for payment, Sébastien says that they are in negotiation with their payment agents to get credit cards working again. Currently not possible. I'd like to think that this is a priority for 2022 since it must surely represent loss of income.
Happy to share what I've learnt - and to be corrected if I've mis-stated anything here… Don't think so. Happy Christmas to one and all!