I haven't been able to replicate this but a few users have reported it and got some info from one of them. They were using Firefox 133.0.3 on a Windows machine, no extensions and not testing API v2.

They went to https://www.inaturalist.org/observations?native&place_id=1&taxon_id=47126&view=species and saw this on every species as they scrolled down:

They sent me console info, which is a lot of repetitive stuff that looks like this:

Experiment add-an-image-to-pdf-with-alt-text-rollout has unknown featureId: addAnImageInPDF RemoteSettingsExperimentLoader.sys.mjs:709:21
The resource at “https://api.gbif.org/v1/map/density/tile?x=6&y=3&z=3&type=TAXON&key=3039459&resolution=4&colors=%2C10%2C%23F7005Ae6%7C10%2C100%2C%23D50067e6%7C100%2C1000%2C%23B5006Ce6%7C1000%2C10000%2C%2394006Ae6%7C10000%2C100000%2C%2372005Fe6%7C100000%2C%2C%2352034Ee6” was blocked by OpaqueResponseBlocking. Reason: “after sniff: content-type declares image/video/audio, but sniffing fails”.
The resource at “https://api.gbif.org/v1/map/density/tile?x=6&y=4&z=3&type=TAXON&key=3039459&resolution=4&colors=%2C10%2C%23F7005Ae6%7C10%2C100%2C%23D50067e6%7C100%2C1000%2C%23B5006Ce6%7C1000%2C10000%2C%2394006Ae6%7C10000%2C100000%2C%2372005Fe6%7C100000%2C%2C%2352034Ee6” was blocked by OpaqueResponseBlocking. Reason: “after sniff: content-type declares image/video/audio, but sniffing fails”.