How do I turn thumbnail previews on and off.

Tabs...they look very much alike don't they, and it's easy to forget what's in each one (especially if they have the same title). We've solved this by placing a recent thumbnail image in the hover menu.

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To turn the thumbnails on and off go to Settings > General > User Interface > Tooltips > Show thumbnails in tooltips > Toggle ON/OFF.
The thumbnail is displayed even when an app/tab is sleeping, so there's no need to wake it to remind yourself of its contents (therefore saving time and CPU).
Hover over a sidebar group and you'll see thumbnails for the three most recently used apps, and also for any app tabs. Click on any thumbnail preview to navigate to that app.