Youtube – How to force YouTube to play videos in their lowest resolution

I don't believe that there is a setting in YouTube to do this. You can achieve this by installing this GreaseMonkey script though.

YouTube HD Suite

Script Summary: Perfect package to enjoy HD videos in YouTube. Always watching or downloading the highest quality format ( HD 1440p / HD 1080p / HD 720p / HQ FLV / MP4 iPod ). Add download icons in video list page.

Version: 3.4.1

To change the setting fort this, follow the steps below:

1. Go to your Settings page.
2. Click the Playback Settings link.
3. Choose between the default setting and the “I have a slow connection” setting.
4. Click the Save Settings button.