Save-Data: on Web extension for Firefox
Sends a standard signal to every website that you wish them to fulfill the request using as little bandwidth as possible. Supporting websites may reduce image quality, avoid auto-playing videos, and take other measures to lower their impact on your data quota.
Learn more about the Save-Data HTTP client hint.
Please note that this extension is not a data compression proxy like the Data Saver option in Google Chrome, or Opera Turbo and Yandex.browser Turbo mode.
The extension is available for Firefox for computers and Firefox for Android. The extension is free and open source software. You can get the source code and report issues on GitHub.