I kind of think you almost answered your own question, what did you change?

I can suggest something that would almost certainly be better than what you had.

Install the "Stylish" extension then you can install a style to provide tabs with color borders.

• Stylish :: Add-ons for Firefox
  https://addons.mozilla.org/en-US/firefox/addon/stylish/
• Tab Color Underscoring active/read/unread (Fx3.6 ) (for 3.6 through 6.0+)

  Colors the bottom border of a tab with RED for active tab, BLUE for a normal tab, GREEN for a still unread tab, MAGENTA(FUCHSIA) for loading/reloading tab. In other words distinguishes tabs as active tab, viewed tab, unviewed tab. Does not affect the background color of a tab so that color usage on the tab will prevail. Bottom border color will show up even if tabs on the tabs bar have been squeezed down so much that only the favicon can be seen. (bottom color is not showing up on reduced height tabs in 4.0 and above).

  You must also obtain install the TabVisited extension from dickvl's MozillaZine user page in order to use this style (link to his page is in the documentation)

  http://userstyles.org/styles/24728/tab-color-underscoring-active-read-unread-fx3-6

Additional Styles you would probably find useful

• Bookmarks Toolbar Fx4 Blue/Folders, Red/Bookmarks - Themes and Skins for Browser (this is what I use puts them all on one row by eliminating icons)

  Sets maximum bookmark width to 36em, if you think that is too wide modify the code to perhaps 170px
  http://userstyles.org/styles/46947

• Scrollbar Menu - Themes and Skins for Browser (also does bookmark menus) - userstyles.org
  http://userstyles.org/styles/52
• Scrollbar Context Menu - Themes and Skins for Browser - userstyles.org
  http://userstyles.org/styles/54

Additional Extensions you would probably find useful

• Stay-Open Menu :: Add-ons for Firefox
  https://addons.mozilla.org/firefox/addon/stay-open-menu/

Thanks dmcritchie. But I have used Firefox for years and the active tab has always highlighted.

  How did that happen without an extension.
  I have been through the whole Windows advanced "appearance" setup

but there is NOTHING I can do there that will affect the TABS in Firefox.

  How could something so simple require such an elaborate cure.
  I don't really want to weigh down Firefox anymore with extensions

and I don't need to color all my tabs.

 I just want to restore the active tab highlight.
I have spent hours fiddling with Windows appearance controls - but nothing affects Tabs.  Select works, but not on Firefox tabs.  Other programs with Tabs still

have active tabs highlighted.