Firefox gets Multi-Process Plugins from Chrome

Tabs Open Relative: https://addons.mozilla.org/en-US/firefox/addon/1956 might be what you're looking for.

There is a lot more to the Chrome tab interface than that - the way they don't resize while you're closing them en masse is particularly slick.

FWIW, TabMixPlus https://addons.mozilla.org/en-US/firefox/addon/1122 can do what Tabs Open Relative does, and much more.

That's plugins (like Flash player), not extensions.

Firefox 3.6 (currently in Beta 4) opens tabs with the same ordering that Chrome does.