Tab previews are in the works for a future release of Mozilla Firefox. In current versions of Firefox you hover your mouse over a non-active tab (i.e. any
I don't see the point personally but I enabled it just because and while it does work it's currently very slow for me. It takes around 1 sec for the preview to appear so finding something by moving the cursor quickly across the tabs is impossible unless you slow way down.
To control how fast/slow tooltips appear modify browser.tabs.cardPreview.delayMs. This is set to 1000 (milliseconds) by default, meaning tooltips only appear once you’ve hovered over a tab for at least a second.