You can combine in usserChrome several instructions as long as have syntax correct.

• https://www.userchrome.org/ Site Is from a Contributor here.

Just a WAG (wild-assed-guess) - conflicting scripts?

Do #1 and #2 work separately? IOW, when either is used separate from the other. Not something I would be interested in, so I didn't test that combination of codes.

#TabsToolbar { /* tab bar */
 -moz-box-ordinal-group: 3 !important;
   }

That moves the tab-strip down

#PanelUI-button {
  -moz-box-ordinal-group: 0;
  border-left: none !important;
  position: absolute;
}

then with #2 you move the PanelUI the where the tab-strip used to be.

Try to use only this code to move the "3-bar" menu button to the left end and leave out the code to adjust the margin-top

#PanelUI-button { -moz-box-ordinal-group: 0 !important; }

Thanks, the-edmeister and cor-el, I will try these tomorrow (hopefully) and re-post (now I'm on a different computer). Thanks for re-formatting the css code, cor-el, as it came out messy when I submitted the text. Yes, both option #1 and option #2 did work separately, correctly. I think the Open Menu (3-bar) is there, as it appeared when I added the FF title bar, and also the Menu bar, but it was otherwise inaccessible, and disappears when the title bar and menu bar are gone.

Thank you the-edmeister and cor-el. Cor-el's solution (see below) was simpler, and works better. The-edmeister, your solution worked partly, but I also needed to add a blank space to ensure other icons did not overlap with the Open Menu button. Cor-el's line also combines well with the CSS code to move the tabs to below the address bar and the bookmarks toolbar. And I discovered how to change the color of the Open Menu button as well! Many thanks for your help. If anyone can advise the CSS code for the Overflow Menu double arrows, I will really be sorted (Presently they hide under the X close window, unless the title bar or menu bar is open). My resultant combined code is below.

/* Tab bar below Navigation & Bookmarks Toolbars - For best results, show Title Bar or Menu Bar */

 #nav-bar { /* main toolbar */
 -moz-box-ordinal-group: 1 !important;
 border-top-width: 0 !important; 
  }
   #PersonalToolbar { /* bookmarks toolbar */
  -moz-box-ordinal-group: 2 !important;
  }
  #TabsToolbar { /* tab bar */
-moz-box-ordinal-group: 3 !important;
  }

/* Move the Open Menu, 3-bar button to the left & change its color */

 #PanelUI-button {
  -moz-box-ordinal-group: 0 !important; 
  color: blue;
  }

PS For anyone needing to know, the userchrome.css file sits in the chrome folder inside your firefox profile.