I don't recall which site it was for me recently, perhaps Sainsbury's, but I reached a card payment screen and it just would not accept payment - it kept getting stuck with a spinner making no progress at the payment stage. Opened exactly the same page in Safari and it worked fine quickly. Next time it happened again, and I remembered that I needed to switch browser.
That payment problem is fixed now though. It was broken for a few months and is fine now.
There's also no way to be sure it was FF in general and not the particular version(s) of FF Beta I was running at the time.
There is no way to know which of numerous factors might contribute without running from a blank profile, which isn't something anyone would do unless really keen on investigating a problem, as it's easier to just run a different browser instead.
If a payment process gets stuck due to an FF-specific adblocker, I'd still count that as an accident of implementation affecting FF users, because final stage payment processes don't have any reason to call up ad-blocked services, and most FF users use an adblocker, don't they?
I couldn’t determine why; I have customized perhaps a dozen variables within Firefox so it is hard to know wether iCloud depends on any of them.