"Non-complicated" is subjective but you can certainly do it with custom css.

Open NotePad and paste this into it:—

.tabbrowser-tab[selected] .tab-content {
  background-color: #D3D7CF !important;
  color: #000000 !important;}
.tabbrowser-tab:hover:not([selected]) .tab-content {
  background-color: #888A85 !important; }
.tabbrowser-tab:not([selected="true"]):not(:hover) .tab-content {
  background-color: #555753 !important; }

Save it as userChrome.css (all lower case apart from the C in the middle) and check it really has and not userChrome.css.txt. Right-click it and open Properties to see its full name. Correct it if needed.

In Firefox open about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true.

Now go to about:support. Look down the blue block on the left of the screen that will open until you come to "Profile Directory". Click the "Open Directory" button next to it. This will open your Profile Directory in Windows Explorer.

Make a directory/folder in that called chrome, all lower case. Drag the userChrome.css you made earlier into it.

Restart Firefox for it to take effect.

You can fiddle about with the colour codes to your taste. The screen shot below shows what the ones given look like — Active Tab: Light Grey, Inactive Tabs: Dark Grey, Inactive Tab being hovered on: Mid-Grey.

Thanks for the info. I'll get to it over the next couple of days and report back to mark solved or request further assistance.

So that I may I be more inclined and more able to help myself in the future do you have any pointers towards documentation for tweaking the Firefox GUI?

If you do a web-search for something like "Firefox custom CSS" or something more specific like "Firefox custom CSS tab colours" there's loads. However check any that you are interested in is fairly recent as what used to work on older versions may not now, at least not without tweaking. Likewise what works now may not in the future.