It cannot be done from extensions since Firefox 57 (since Quantum) and higher, but there is a workaround for this:
https://support.mozilla.org/en-US/questions/1185935
Also on my project site (but I'm not sure if you will have access to it without registration):
https://gitlab.com/kroppy/TreeTabs/issues/40

Mozilla developers are working on API to allow it. Once this new API will be implemented in Firefox I assure you, I will add a dedicated button to hide original tabstrip.