"open a new tab" is not a tab, it's toolbar button and has nothing to do with TMP, it's an Fx button. Changing its color will do you no good as it's an image and you need to change the image. You can do this by using
Stylish or
userChrom.cssYou can do the same for tabs (height, gradient) in the same manner.
If you need help with that, i can help you but you can also search the Stylish site for styles and post in the
forum if you need help with that.
For instance, New tab button color:
#new-tab-button, .tabs-newtab-button
{ -moz-appearance: none !important;
background-color: red !important; }

But you can change the image to your liking with something like this, for instance:
Code:
#new-tab-button, .tabs-newtab-button
{ -moz-appearance: none !important;
list-style-image: none !important;
background: url(data:image/png;base64,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) center no-repeat!important; }

You can also change the bg for that image but it will take some tweaking to get it right.
There many styles available for tabs. You can use a code like this to change colors for tabs and New tab button:
.tabbrowser-tabs tab, #new-tab-button, .tabs-newtab-button
{ -moz-appearance: none !important;
background-image: -moz-linear-gradient(top, #ccc, #4E5A6C, #25456C)!important; }

The tabs height can be changed with something like this:
.tabbrowser-tab
{ -moz-appearance: none !important;
height: 21px !important; }
I don't think TMP should fiddle with tabs height 'cause there's just too much that can go wrong, like multi-row display, diff. themes, personas,etc.