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[spitsong]

re: Firefox

From Firefox 2.0.0.3 (on a PC!), click Options from the Tools menu. In the Startup section of the Main pane, select "Show my Windows and tabs from last time" rather than what's already being displayed.

I imagine the Mac version works similarly, though it's likely to be under a Preferences menu item (the Mac standard), perhaps under the Edit menu, rather than an Options menu (the oft-equivalent PC standard).