You can try Vertical Tabs in Firefox now
Mozilla is working on bringing vertical tabs to the Firefox web browser, and you can try the feature right now, if you want.
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it needs to collapse automatically, otherwise it just wastes a lot of space
Ideally, a toggle for auto collapse. With my ultrawide monitor, I’m keen for the sidebar to always be visible. But if you want it to collapse, that should be available as well.
If you want collaspable vertical tabs, you should look at using sideberry & this userChrome.css file set: https://github.com/Redundakitties/colorful-minimalist/tree/main
I am using this https://github.com/lucaspar/firefox-vertical-tabs/ ; same idea