Not Piro's fault for the top tabs, that is Firefox's new API limitation and Piro has gone to great lengths to get it this far. Just a simple couple of steps to get rid of the top tool bar if you read the bubble information in the new update or just look for the links in these reviews.

Still an indispensable addon for Firefox. I like the new version and thrilled it made it over to the new version 57. I wouldn't use Firefox without it to be quite honest.

Thank you Piro, been using your addons for years. My workflow would not be the same without them.

For those that can't be bothered to educate themselves, here's a copy/paste from someone that took the time to write out step by step instructions. Takes less than 2 minutes to do.

If you're like me and primarily used this add-on to remove the tabs from the top, follow these instructions provided by user Soulyaris:

"Inside your profile folder(it usual c:\Users\*user*\AppData\Roaming\Mozilla\Firefox\Profiles\*something*-default (check date of modified),
Create a new folder named chrome.
Inside the chrome folder, create two new files, userChrome.css and userContent.css.
At the top of userChrome.css, add (copy/paste) the following:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
/* Hides tab bar*/
#TabsToolbar { visibility: collapse; }

Save it and restart firefox. Enjoy."

That's it, enjoy tree-style without the top tabs.