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Mozilla released a Firefox Nightly test build with vertical tabs - gHacks Tech News
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Just the hide tabbar button would be amazing when combined with the existing vertical tab extensions. I know you can use custom css to hide the tab bar (that's what I do), but the option would make it accessible to regular users too.
Sounds like that's in here:
"The test build shows the horizontal tab bar and the sidebar at the same time by default. A click on the new "hide tab strip" button hides the horizontal tab bar so that only the vertical sidebar remains."