Just upgraded today to firefox 58.0.2. Deleted my old tab mix plus extension and installed the new webextensions (experimental) version. It shows up in my add-ons page where I can enable/disable it. However, the icon for this tabmix does not appear anywhere in my toolbar. Even looked in the overflow panel (>>). It's nowhere to be seen and hence is unusable at the moment. Hence, giving it 4* in hope that this will be remedied soon. This used to be my favorite addon for firefox. But mozilla destroyed this single best thing for firefox. If this extension works again, I will change my rating (or add new 5* rating again).