selected (single click) what I wanted to add to my toolbars in the mail window. The "save option" was greyed out. How do I save these to my toolbars?

Just selecting (clicking) the button is not enough.

The following link might be helpful: Unified Toolbar https://support.mozilla.org/en-US/kb/unified-toolbar

Quotation from the above link:

2. This is the Universal Toolbar for the Space you have selected that you are customizing and its current buttons. Drag and Drop to move the buttons around, or use the Context Menu (control click on macOS, right click on Windows and Linux) to move the buttons. You can also use the keyboard: Tab to select a button, Delete to remove it, and Alt with arrow keys to move buttons around on the toolbar, and Enter to add a button to the toolbar.

If you drag and drop to move the buttons, please make sure that you drop the buttons in area 2 shown in the picture in the above link.

If you follow the instructions above, the "Save" button will become clickable.

I found the problem. Instead of left-clicking and then trying to click "save," I had to right-click on the icon, then select "add to main toolbar;" and then I was able to click "save."