Not yet that I know of. Someone will come along though.

Copy/Paste about:addons into the Address Bar then Enter You will be at the Legacy page, there is a Button there use that to look.

Web Extensions are evolving and many of them are changing to be compatible with the new Firefox 57. Tracking them here: http://bit.ly/2j0Anlj. Click this link then inside the is a link to it's search.

Please let us know if this solved your issue or if need further assistance.

Thanks for the info. But i really need to find out a substitution for Tab Mix Plus. Or firefox can add the multiple row of tabs option to the default setting.

OMG, I returned from Chrome to Firefox in order to have multiple tab rows--only to find Tab Mix Plus doesn't run on FF 57. Why is this even an add-on rather than a builtin option?

You can force Firefox to wrap the tab bar onto multiple rows using a custom style rule. There are various threads about this, but here's one way to create a multi-row tab bar:

(A) Create a new chrome folder in your profile folder. This article has the steps for that (#1, #2, and optionally #3)

https://www.userchrome.org/how-create-userchrome-css.html

(B) Download the following file and move it into that chrome folder (creates up to 5 rows of tabs):

https://www.userchrome.org/samples/userChrome-multi_row_tabs.css

(C) Rename that file to just userChrome.css

This assumes you do not already have a file named userChrome.css. If you do, you can combine their contents together so you can keep all of your current custom style rules.

The next time you quit Firefox and start it up again, it should discover that file and apply the rules.

Comparison screenshot attached for reference (Windows 7).