Firefox has a helpful feature that presents thumbnail previews of all your open tabs, but you have to go into the 'about:config' interface to do it. This interface comes with warnings that it might mess with your browser's performance, so don't change anything without knowing what it does. To turn this feature on, simply follow these steps:
1. Type 'about:config' into the Location bar and press Enter 2. Click on 'I'll be careful, I promise' 3. Type 'browser.ctrlTab.previews' into the Search bar and press Enter 4. Double-click on the entry to change it to True 5. Exit the 'about:config' window
Now, hold down the 'Ctrl+Tab' keys and you'll be presented with thumbnails of all your open tabs, which you can navigate through using the arrow keys or your mouse.