I think it is because we still use the permission model for `clipboard.read()` which only work on addon, https://searchfox.org/mozilla-central/rev/24c1cdc33ccce692612276cd0d3e9a44f6c22fd3/dom/events/Clipboard.cpp#59-61. We are working on a new model that show a paste button instead in bug 1744524. `clipboard.read()` will be applied to the same model.
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I think it is because we still use the permission model for `clipboard.read()` which only works for addon, https://searchfox.org/mozilla-central/rev/24c1cdc33ccce692612276cd0d3e9a44f6c22fd3/dom/events/Clipboard.cpp#59-61. We are working on a new model that show a paste button instead in bug 1744524. `clipboard.read()` will be applied to the same model.