After about a minute of idle time, the plugin came down and everything works. I think the bug here is that attempted use doesn't trigger the download, and there's no UI. My guess is this has been this way for ages so not a recent regression. We've had a long standing project to improve the UI revolving around gmp plugin install that would help here.
Bug 1766037 Comment 8 Edit History
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After about a minute of idle time, the plugin came down and everything works. There are two bugs here - 1) attempted use doesn't trigger the download, and 2) no UI. My bet is this has been this way for ages so not a recent regression. Media team has a backlogged project revolving around improving gmp plugin UI that would help here.