Firefox: Vertical tabs are here!

With the demise of Arc Browser, fear not, vertical tabs are finally here in Firefox (with a little help).

It's built and deployed, just not yet set loose on the masses. But all it needs is a little tweak.

• Go to about:config
• set sidebar.verticalTabs to true
• questionable: sometimes setting sidebar.revamp to true is also necessary, but in my case it was automatically set.