Vertical tabs via extension and userchrome mods in Firefox sadly don't have the polish compared to MS Edge or Vivaldi.
It's fine, but I find it clashing with the browser history sidebar, and being tuned off by default in private tabs.
It can come close, but until (if ever) official support will be provided, It'll always feel like a bandaid fix to me.
For that reason I keep both vertical and horizontal tabs enabled. On other browsers with native support I never missed them, in Firefox however I need both. Relying on sidebar-powered vertical tabs alone is not a good User experience.