I know this is old news by now, but I was not sure how to activate vertical tabs on all of my browsers. It certainly is not obvious, so I wanted to include the instructions here in case anyone else wants to try it out.
Type about:config into the address bar of Firefox. You’ll be greeted with a warning that you are accessing advanced settings - click “Accept the Risk and Continue.” Search for the boolean preference sidebar.verticalTabs and set to true. That’s it! Enjoy your vertical Tabs!
Edit: You may also need to set sidebar.revamp to true if it is not already/automatically set.
They take too much screen space, and my muscle memory is built on horizontal tabs, 2500 of them
They take up the mostly-unused horizontal screen space, though, whereas the vertical space that it frees up is far more precious to me. And I get to see most of the tab titles as a bonus!
I used some css tweak to display a Sidebery on mouseover. But after the latest Firefox update it broke.
On my 1080p display it’s taking up too much space, but on an ultrawide it’s perfect use of the additional real estate.
I have four 40" cheap 4k tvs, I split them in roughly 6 sections