I tried it in both Chrome and Pale Moon and got the same results. I haven't used Firefox ever since it became trash and I'm not going to install it just to test this... But I truly doubt the server is configured to alter the results based on the browser.

1) The Firefox results definitely look wrong. 270987 should be the first post in those results. 276838 was never tagged "animated" and that tag is not listed in the results you posted either. Are you sure the page loaded? Those aren't just the results for the previous search you tried?

2) Bear in mind that for many posts, the original file will be JPEG. The original for 276838 is JPEG so naturally you will only get JPEG urls for that one.

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Edit: Please don't create a new forum topic for every question or bug report. The correct topic to report bugs in is here.

Edit 2: I've re-purposed this topic for general discussion of the API. If you have any further questions, please ask them here.

Edit 3: I had the browsers in the image backwards. I've edited the response above accordingly.