This extension writes the tab number to the tabs, the ones accessible with ctrl/cmd + number
What is Tab Numbers?
Tab Numbers is a Firefox add-on that enhances tab navigation by displaying tab numbers on each tab, enabling easy access and navigation using control or command followed by the tab number.
Tab Numbers is risky to use as it requires a number of sensitive permissions that can potentially harm your browser and steal your data. Exercise caution when installing this add-on. Review carefully before installing. We recommend that you only install Tab Numbers if you trust the publisher.
Tab Numbers has earned a fairly good reputation and likely can be trusted.