Musescore Downloader for Chrome
More About Musescore Downloader
For the extension to work, you need to have your browser window maximized. If something isn't working, you might have to reload the Musescore website or even restart the whole browser.
If you still have any problems, create an issue in the linked GitHub repository or on the user support page. Please include extension version, browser, operating system and describe the error in detail.