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As "Julien" wrote incorrectly the WHOLE "BarTab" functionality is NOT implemented by default in Firefox 4.
The mentioned about:config preference "browser.sessionstore.max_concurrent_tabs" lacks of configuring the unload behavior. You can't configure after what time the tabs should unload automatically. And you can't configure which sites (hostnames) have to load always.
For some users the about:config preference maybe sufficient. But it's not the whole Bartab functionality.
So we have to wait for the new version. Have patience. It's free and it is done when it is done. :-)