I bet you didn't know that you can reopen a Firefox tab you have closed. Let's say for instance you closed a Firefox tab, but remembered you needed information from it immediately after closing it, or possibly you closed it by accident.
1. Click the Red "X" or type Ctrl+W to close this tab.
2. Right click on one of the other tabs and select [Undo Close Tab].
3. And Yes! It's back!