This problem is being investigated by Adobe.
Some people have reported that it might be an extension blocking traffic to Adobe, such as an ad blocker. They were able to upload images when using private mode/incognito, using another browser, or turning off ad blockers on Adobe pages.
You can try turning off ad blockers on Adobe Firefly. For example, if you use uBlock Origin, try turning it off on Firefly. Click the uBlock Origin icon in your menubar and turn it off:

Or if you are using Ad Bl
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