There are keyboard shortcuts for opening Bookmarks or History in the Firefox sidebar, but the Synced Tabs sidebar lacks a corresponding shortcut. See the following screenshot:
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Additional, related shortcut feature request: In the Synced Tabs sidebar, when I right-click a device, there's a shortcut for "Open All in Tabs". But I'm not aware of a way to use this shortcut without having to right-click the device first. It would be neat to have a globally available keyboard shortcut which directly opened all synced tabs in a new tab, without needing both a mouse click and an open Synced Tabs sidebar.