Actually, this is secretly built in. :-)

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste ctrl and pause while the list is filtered

(3) Double-click browser.ctrlTab.previews to set its value to true.

Now try Ctrl+Tab and, like Alt+Tab, release the tab but keep Ctrl down. Feel familiar?

This works on a per-window basis for this window and other windows opened going forward.

Unfortunately, on my system, only 6 thumbnails fit, but for purposes of going back to the previous tab or two, it's probably sufficient. What do you think?