This works on the LTS version of Firefox but needs an update if you’re running the latest version.

Type about:config in the address bar and press Enter to load it.

In the search box above the list, type userprof and pause while the list is filtered. If you do not see anything on the list, ignore this step.

Switch toolkit.legacyUserProfileCustomizations.stylesheets preference from false to true.

Type about:support in the address bar and press Enter to load it.

Note the Profile Directory.

cd to Profile Directory and create a folder called chrome.

***

mkdir chrome;

cat << EOF > ./chrome/userChrome.css #TabsToolbar { visibility: collapse; }

EOF

***

In Firefox again:

Restart all Firefox sessions.

Right click anywhere menu section of Firefox, and check 'Menu Bar'.

... and that ridiculous kluge is why the vast majority of users will never experience vertical tabs - which would be a better experience for the vast majority of users...

... and also a great reason for keeping notes! As a linux user modifying a text file is par for the course, yet I totally agree in substance with your point.

Been using this as my userChrome.css:

https://gist.github.com/BrianGilbert/1ad7e3931406f485a86a35a...

Thanks. Surprised there isn't an extension to allow toggling on/off.

Regretfully Mozilla hasn't gotten around to providing the API for that.

Mozilla seems to see potential for abuse with this, which is why they are not offering an API for this.

Could just be an about:config flag, that would be better than having to apply userChrome.css on every profile where you need it.

I think it is because firefox wants to keep the same extention api with chrome.

The only way to hide the horizontal tab is with userChrome.css