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Mozilla released a Firefox Nightly test build with vertical tabs - gHacks Tech News
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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I think you're missing what's going on. The text is still written left-to-right. You don't need to read the tabs vertically. The tabs are stacked on top of each other in the sidebar instead of lined up along the top of the window.
I believe their point is that their language is left to right, so it just makes sense to them to have the tabs structure left to right as well.
I happen to share this sentiment, but can understand why some people may like it different.