Since last September, you could set
browser.sessionstore.max_concurrent_tabs to 0, and you would essentially have a built-in BarTab. I slipped that in later in the Firefox 4 release cycle as a part of cascaded session restore, but it required going into
about:config and changing a preference value (or installing something like BarTab Lite).
Starting with the latest nightly (and soon to be Aurora 8),
browser.sessionstore.max_concurrent_tabs is no more. We’re no longer allowing you to specify a specific number of tabs to restore concurrently. Instead we now allow you to either restore on demand, or use the hard-coded 3 tabs at a time value. The new preference is called
browser.sessionstore.restore_on_demand. If you had customized
max_concurrent_tabs and set it to 0, then
restore_on_demand will be migrated to true. Unlike the old preference,
restore_on_demand is exposed in the Preferences/Options dialog to make it accessible to a larger audience.
For the details, check out bug 648683.