With Firefox 2, Undo Close Tab is a standard feature. While you can undo a close tab by right-clicking on the tab bar and selecting ‘Undo Close Tab’ from the context menu. However, this reopens the last closed tab and you will have to repeat this until you have reopened the desired tab.
Tip: Hidden in the History Menu is the option, Recently Closed Tabs. When selected, it opens a sub-menu with a list of all your recently closed tabs. Simply highlight the tab you wish to reopen or you can select ‘Open All in Tabs’.