My own personal Seamonkey userstyle that removes many of the unused buttons n' stuff in Seamonkey.
For safari-style tabs that utilize as much width as possible equally, in about:config, set browser.tabs.tabMaxWidth to a very large number, and browser.tabs.tabMinWidth to a very small number.
As you can see, many changes were made to remove buttons and panels that I deemed unnecessary. The tabs were also cleaned up to remove the borders and other things. Works best with Helvetica and macOS, but CSS is, of course, universal. Compatable with Seamonkey 2.46-2.48 and probably a few other versions below it.
I am using the Orthodox theme by John Duncan in that screenshot, but the only place it shows up is in the favicons.