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You can try Vertical Tabs in Firefox now
(www.ghacks.net)
A place to discuss the news and latest developments on the open-source browser Firefox
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