Tab Mix Plus is one of the most popular addons for Firefox. Also it is among the most popular ones that has still not been upgraded to work with FF3.
Now it is a very powerful addon providing many features - like duplicating tabs, session management, undoing closed tabs, modifying the opening behavior to open new tabs to the right of originating tab.
Since I could not upgrade it with FF3, I was disappointed at first. However, it is not a good enoug reason for not upgrading to FF3, moreover after browsing a while I got to know certain features and workarounds - which surprisingly let me have all the features of Tab Mix Plus which I loved this addon for!
So without further ado, here's how to live without Tab Mix Plus.
Undo Closed Tabs
I was surprised to learn Firefox can already do it. However, if you had selected Tab Mix Plus to manage the sessions instead of Firefox, you won't be able to do it.
To enable Firefox session management, point your browser to about:config, ignore any warnings that Firefox may give - the warning is for novice users only which you are not - but needless to say don't screw around with the settings too much.
Find browser.sessionstore.enabled, and set it to true.
That's it, now restart Firefox, and you're done. To undo closed tabs, you can either go to History -> Recently Closed Tabs, or press Ctrl+Shift+T. If you really fancy, you can also install the Undo Closed Tabs addon which will add a menu item and a button for the same task.
Additional Tip: You can set browser.sessionstore.max_tabs_undo to something like 99 to remember more history.
Well here you go: Duplicate Tab. This addon provides exactly the same functionality.
Maintain Tab Order
Here it is: Tab Control. This will arrange the newly opened tabs beside its parent instead of far right.
These are the main features that I have been using from Tab Mix Plus. Also I'm fairly sure that if there is anything else that you've been using, you'll find a workaround for that too if you google a bit. So happy surfing with Firefox 3!