The standard reply to this complaint is "use chrome.css", which sure, you and I can... but Mozilla has repeatedly threatened to remove support for it altogether

The standard reply to this complaint is "use chrome.css", which sure, you and I can... but Mozilla has repeatedly threatened to remove support for it altogether. I don't even have to write the css, I can just use https://github.com/ranmaru22/firefox-vertical-tabs https://github.com/ranmaru22/firefox-vertical-tabs, but in Edge, I can just select the option... not install an extension, click things in a hidden page, make a directory deep in $APPDATA, put a CSS file in that directory, and then also paste more CSS into said extension's custom CSS field, then restart browser.