hi, the + button at the end of the tab bar in order to open a new tab was always there by default... you can remove it when you go into customization mode though: Customize Firefox controls, buttons and toolbars

Do you mean you want it to be smaller and maybe a little lighter in color like it was before? For example, the icon is 16 pixels by 16 pixels. If you were to reduce it to 10 pixels by 10 pixels, it would be about the size of the x close button on the tabs.

There is no built-in feature to make those kinds of changes, but many parts of Firefox's interface can be modified by apply custom style rules. For example, this one makes the size change and also "fades" it by making it less opaque (more transparent):

  #TabsToolbar .tabs-newtab-button .toolbarbutton-icon {
    height: 10px !important;
    width: 10px !important;
    opacity: 0.6 !important;
  }

I have attached a "before and after" screenshot showing the effect. What do you think?

The screenshot shows the Stylish for Firefox extension, which is very handy for testing style rules. Unfortunately, it won't work in Firefox 57+ so it's not a long term option. Instead, you can add custom style rules to an optional file that Firefox reads at every startup called userChrome.css (this file doesn't exist unless you create it). That is more work to set up, but has a longer future. When you have five minutes, take a look at the steps here: https://support.mozilla.org/questions/1164362#answer-978078

Actually this is closer to Firefox 55:

  #TabsToolbar .tabs-newtab-button .toolbarbutton-icon {
    height: 12px !important;
    width: 12px !important;
    opacity: 0.7 !important;
  }