I didn't want to go into details, but I think we actually get Netflix in 1080(p?) using either Firefox or Chrome, but Amazon Prime looks like 720p using Firefox.
EDIT - From their website (https://help.netflix.com/en/node/23931):
Resolution Windows computers support streaming in the following browser resolutions:
Google Chrome up to 720p
Internet Explorer up to 1080p
Microsoft Edge up to 4K*
Mozilla Firefox up to 720p
Opera up to 720p
Windows 8 app up to 1080p
Windows 10 app up to 4K*