for your synch purpose on iCloud i think Safari is your best bet. it will be seamless on apple products. only draw back is lack of add ons. safari takes some getting used to but is good enough once you do.

firefox has got to bloated and can slow your systems down. sure you get some great add ons but once you wean yourself off these, and i bet you find a heap you first installed cause they looked good but haven't used in a while, then you won't even miss them.

so if only comparing the 2, and across apple products synching is the criteria, Safari it is.