Just a standard HTTP request in a browser which is essentially identical to performing it via cURL or other HTTP Library.
curl -H 'Accept: application/vnd.twitchtv.v5+json' \
-H 'Client-ID: uo6dggojyb8d6soh92zknwmi5ej1q2' \
-X GET 'https://api.twitch.tv/kraken/videos/370746236'
(Just with a real client ID)
Might be an issue with your JSON decoder, but there’s nothing special in the thumbnail section that could throw a wobble.