@TABSbelowURL: "Users are asking for 10 years, why are you that deaf ???"
@stevie22b: "All these years on and I still cannot understand why the dev's simply do not make that an easy option for us?"
It's worse than that. The functionality USED to be there, when they first moved the tabs above the navigation bar. It used to be in about:config. The devs deliberately took that option away, and have spent the last 10+ years trying to keep us from using userChrome.dss from restoring the tabs below the nav bar.
How petty do you have to be, when people prefer a particular configuration, to spend more than a decade deliberately trying to code it away? "Oh, they don't like the way we decided it should be laid out, so we'll just FORCE it on them."