The UndoCloseTab Firefox extension saves you from trigger-happy clicks that close tabs you meant to keep open.
The extension adds a "Undo Close Tab" to the right-click tab bar context menu which will restore the last tab you closed. I love Firefox extensions that save you from losing your place. This coupled with previously-mentioned SessionSaver can keep you from searching through your history to root out those web pages you'll need again.
UndoCloseTab [via My Moustache]