Don't worry, Firefox often gets blocked by the Google reCaptcha v3 system that FurryNetwork is using, too. It's the same system that's blocking your Opera browser. Also, I don't think they read these suggestions anymore, I haven't seen a reply from site staff in over a year now.

It's almost certainly due to the new Google reCaptcha v3 they are now using in the login page of FurryNetwork. It's very strict about browsers and browser settings, and the newer versions of Firefox have a lot of privacy options enabled by default now, which are caught by the new reCaptcha v3 system and flagged as "risky" so your login gets blocked. If you want to continue using FurryNetwork, you may have to disable Firefox's privacy settings - just another way Google is screwing anyone not using their browser libraries, and we've had posts here for over a month now asking the FurryNetwork devs to correct the issue with no responses.

I tried setting up a localhost Docker image with a minimal Angular 10 UI to proxy API calls (using my cookies and localStorage settings from my local browser) and their CORS policies are set so strictly that even a local.furrynetwork.com host config was not accepted by their API. Every indication seems to me that they don't want third parties to use their API at all, I guess? If you have an even semi-public API, you don't screw down extremely strict CORS policies like that - they are not default in most platforms (and certainly not in the PHP version and framework FurryNetwork is using).

That's not unique to Pixiv. InkBunny, E621 and other sites (most booru's) also support that feature.

Hey folks, the issue stems from FurryNetwork making use of the Google reCAPTCHA v3 system, which analyzes your IP address and browser history to determine "how real" your client is. It assigns very low scores to automated systems (like PostyBirb) and to systems coming from IP addresses which Google finds "questionable" as being used by humans (based on previous traffic, and known server/VPN/gateway IPs). When this happens (FurryNetwork rejects your login due to a low reCATPCHA score), you don't get an error message telling you that your login was rejected due to looking like a bot, instead you just get the generic "Invalid username and password combination" message. Sadly there is no way to fix these issues, which is why I've requested API Key access for API Developers (this is something the PostyBirb folks could also easily accommodate in their app). I'd recommend finding my API Key Feature Request and up-voting it in hopes it will get some attention.

I had been using your API for a couple years before you added reCAPTCHA to your system in Dec, 2019. Now there's no reliable way to login in order to consume the API in an automated fashion. Could you please either allow us an API endpoint for automated login or provide API Keys for developers to authenticate and use the API? Would be much appreciated.