I think the Flickr pro account is very good value.
I have used the FREE FLICKR in the past but it does keep showing warning messages about not having the latest version of browser [Firefox]. Not surprising as that is not compatible with the older versions of OSX.
I have looked through the FLICKR PRO web info and cannot find any statement of System Requirements.
There does not seem to be any way of finding these before purchase.
But I could be wrong.
Maybe others have been able to locate them.