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Mozilla released a Firefox Nightly test build with vertical tabs - gHacks Tech News
(www.ghacks.net)
A place to discuss the news and latest developments on the open-source browser Firefox
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."