No, there is no solution right now.
You need to wait till there are APIs to hide the default tab browser and to add new toolbars, then you would need to re-implement the entire tab browser, just to make this small change.
OK, it’s what I feared… Webext system is clearely not mature enough to replace the old one for now.
I’ll look for alternatives for the next months/years… Maybe firefox clones like Pale Moon or Waterfox… I definitely don’t understand Mozilla’s strategy of killing Firefox’s key features against Chrome…