Has been an extremely useful extension for me for years but as mentioned by Sebastian himself, this feature is now integrated in Firefox and easy to operate in about:config. I should add that the built-in feature actually works much better than the extension. Thanks for your work. No one lives forever.

Does't work on lollipop

I'm using Android tablet. This extension does nothing. All my new tabs go to the end of the line

FireFox 52 = Doesn't work while other windows are being loaded it6 attaches to last LOADING window and Loads the new tab beside it. NOT from the Primary Tab

I think this extension only simplifies the manual inclusion of this option: "browser.tabs.insertAfterCurrent"

This can be done manually using two steps:
1. Type in the address bar: "about:config?filter=browser.tabs.insertAfterCurrent".
2. Set to "true" by double click on it.

Done.

Not working on my tablet

Does not work on Firefox for Android

It was perfect, but now it opens multiple tabs in the wrong order.
According to the author that's the way it should be, despite default Firefox behavior being different.
I can't recommend the extensions that changes something it shouldn't change, and when other alternatives available.