You simply need to click its icon and then choose from the options that appear in a pull-down menu: you can program the tab to reappear later that day, the next day, the weekend, next week or next month. But if you have a specific date and time in mind, you can manually put that in, as well. When the snooze time's up, the tab will pop back into your browser and will even notify you of its presence.
The latest Test Pilot version also comes with Pulse, which sends the browser's engineers your instant feedback on which sites work well and which don't. We're guess that's a lot less exciting than SnoozeTab -- either way, you can test both features as long as you have the Test Pilot add-on. It's an extension for the stable version of Firefox, so you can play with upcoming features even if you don't have the browser's experimental builds.