If you accidentally close a tab in Firefox, fear not. Turns out it's pretty easy to get the tab back with the page loaded, at least it is in Firefox 2. You have several options:

1) Right-click on another open tab (if there are other tabs), and select the 'Undo Close Tab' entry. This will bring back the last closed tab.

2) Go to History -> Recently Closed Tabs and it will show all of your recently closed tabs. Using this, you can open tabs closed long ago.

3) Hit 'Command + Shift + T' on the Mac, or 'Control + Shift + T' on a PC, and the last closed tab will be reopened.