Decent replacement for https://addons.mozilla.org/en-US/firefox/addon/tab-counter/ that works with multiprocess, but the icon and box around the number is distracting. Please add an option to have it display as a number in the same font as Firefox and nothing more.

The icon doesn't clearly communicate the purpose of the addon to me, but that doesn't really matter. Maybe it would be fine if the icon looked more like a tab and less like a line with bends in it.
There is an option to set the addon's icon to transparent and just leave the little thing that says how many tabs there are. It's nice that there is a little popup when you click on the addon icon that tells you how many tabs you have open in the current window and how many you have total.

If I change colour for black, Tab Counter is always in red.

You're welcome. I use Firefox 52.0 x86 and Cyberfox 52.0 x86 (Intel).