You should still see a change of the icon next to a live feed item from a highlighted feed icon to a grayed icon if you've visited that item.
This will however only work if you keep history because feed items are now treated as history items.
If you clear the history then the visited status is reset.

So check that you keep history and aren't running Firefox in Private Browsing mode.

• https://support.mozilla.org/kb/Private+Browsing

You can also check for extensions or custom themes that aren't working properly.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Start Firefox in Safe Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

Thank you very much. It would appear my theme does not support this feature in the latest version of Firefox.

You're welcome

Did you check if there is an update of the theme available or if this issue already has been reported?

Unfortunately there is no available update. And as far as I can tell nobody has reported it. I'm assuming I would find this in the reviews for the theme?

Also, I thought I would mention this is with the Office Black theme.

You can use this to restore some default icons.

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (interface) and userContent.css (websites) are located in the chrome folder in the user profile folder.

• http://kb.mozillazine.org/Editing_configuration

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* default favicon */

#tabbrowser-tabs .tab-icon-image:not(src),
.sidebar-placesTreechildren::-moz-tree-image(leaf),
.bookmark-item:not(src):not([container]):not([type]),
.bookmark-item[container]:not([livemark]):not([query]){
 list-style-image: url("data:image/png;base64,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")!important;
 -moz-image-region: rect(0px,16px,16px,0px)!important;
}
.bookmark-item[container]:not([livemark]):not([query]){
 -moz-image-region: rect(0,32px,16px,16px)!important;
}

.sidebar-placesTreechildren::-moz-tree-image(title,container,livemark),
.bookmark-item[container][livemark]{
 list-style-image: url("data:image/png;base64,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")!important;
 -moz-image-region:auto!important;
}
.sidebar-placesTreechildren::-moz-tree-image(title,livemarkItem),
.bookmark-item[container][livemark] .bookmark-item{
  list-style-image: url("data:image/png;base64,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")!important;
 -moz-image-region: rect(0px,16px,16px,0px)!important;
}
.sidebar-placesTreechildren::-moz-tree-image(title,livemarkItem,visited),
.bookmark-item[container][livemark] .bookmark-item[visited]{
 -moz-image-region: rect(0px,32px,16px,16px)!important;
}

Wow, that worked! That is really cool. Thanks!