Kuroyume: you can get back your accidentally closed tabs with FF2.0 (and there is an extension for 1.5 that did the same). In FF2.0, it's under History:Recently Closed Tabs.

Also, under Tools:Options:Tabbed Browsing:Display Options you can tell FF to show tab closure buttons:
on the active tab only
on all tabs
nowhere
on the tab strip

I have mine set for nowhere, since the best way to close tabs with minimal chance of accidental closure is by middle-clicking on the tab you wish to close.