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Mozilla released a Firefox Nightly test build with vertical tabs - gHacks Tech News
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A place to discuss the news and latest developments on the open-source browser Firefox
I've been using "edge-style" vertical tabs (collapsed to icons by default, expanded with text on mouse hover) for the past 2-3y with a custom CSS. I think expanding on hover is the best trade off between readability and good use of space - I wish Firefox implements it some day.