The tab height is based on built-in style rules. You can override the built-in style rules by adding new style rules to a file named userChrome.css or using the Stylish add-on. (The advantage of using the Stylish extension is you do not need to restart Firefox to see the rules applied, so you can experiment more quickly.) Because the tab contains text, icons, and possibly a close button, it may take several rules to get the size the way you like it. I didn't see a recipe in my quick search, but I'm sure you can find some good suggestions if you dig around.

Thanks!

I do have some entries in my userChrome that are still used but none relating to tabs. Anyway, I did find a solution in the Add-on "Hide Caption Titlebar Plus at " https://addons.mozilla.org/en-US/firefox/addon/hide-caption-titlebar-plus-sma/?src=userprofile

Note the Min, Max, & Close button is styled as a micro-mini that expands on mouse-over... Love it!

Oh, and the orange Firefox menu button is also reduced to an "F" that still functions as always with a mouse-over.