Oneliner with Sidebery by

Firefox CSS theme with oneline and vertical tabs sidebar based on Sidebery.

Required

Firefox/Librewolf

Optional

Sidebery

Usage

Go to about:config in your Firefox and set the value of toolkit.legacyUserProfileCustomizations.stylesheets to true.
Download userChrome.css from /chrome.
Go to about:profiles in Firefox and open your Profile Folder by selecting root directory
Add to Firefox user profile folder under /chrome downloaded userChrome.css.

Optional

Install Sidebery addon in Firefox from Firefox Addon Store.
Download sidebery.json and load sidebery.json it in Sidebery settings.

Credits

MrOtherGuy for his amazing firefox css sidebar autoclose hack -
zayihu -
freeplay - https://freeplay.floof.company/