You can hide the individual close tab buttons using a custom style rule. However, there is not a built-in way to create a new "close current tab" button at the end of the bar like in the old days. There is an add-on to create a Close Tab Button on the main toolbar, and you can move it to the tab bar. So putting those two things together...

Hiding the Close Tab buttons

To apply custom style rules to Firefox's user interface area, you need to create a userChrome.css file in a particular folder. I created a website to help with this: https://www.userchrome.org/. You probably need to allow 10-15 minutes if it's all new to you; take your time.

Here's the rule:

/* Hide Tab Close buttons */
#tabbrowser-tabs .tab-close-button {
  display: none !important;
  visibility: collapse !important;
}

Close Tab toolbar button

Here's the extension: https://addons.mozilla.org/firefox/addon/close-the-tab-button/

Then use the Customize feature to drag the button to your desired position on the tab bar. I'm attaching a screenshot illustrating what that looks like.

I noticed there are a couple other Close Tab extensions now, so if you don't like the look of this button, you could investigate others.

Thanks for your help, much appreciated. It did the job quite nicely.

I really miss the easy customizable features of Classic Theme Restorer, I wish it was avaialable with the current build. There are so many customizations to my layout I have to re-figure out again.

Cheers!