They're both online editors that use the word "remix", but that's really it.

Mozilla Thimble is a HTML/CSS/JS learning tool. It's frontend only and doesn't have realtime collaboration.

Gomix is backend and frontend. It's collaborative, so a team can work on it at the same time. They're looking to change how teams work together by replacing version control. It also deploys instantly, so there's no saving/uploading/pushing/etc.

Source: https://learning.mozilla.org/blog/introducing-the-new-thimbl... and https://www.joelonsoftware.com/2016/05/31/introducing-hyperd...