I vouch that it is less taxing than other add-ons, you can test it by opening "About:Performance" on a different window in a different monitor, and then opening different tabs on a different window. I also suggest highlighting Tab counter plus entry in "About:performance page.

I also appreciate that it doesn't ask for any invasive needless permissions. Kudos for that.

I like that you can choose SVG icon which shows only the number of tabs, however, the font/numbers looked too fine and small with low contrast, I changed both background and text color, but still it was too hard for me to read. Hence, I suggest to add some customization to change the SVG font values like thickness or font size. Not sure if possible, but hope so. Alternatively, add a third option to just show the WebExtension badge without the Tab icon, meaning just the number.

A suggestion, If possible add number of running tabs, meaning tabs that are not suspended. Reason for this suggestion, you can see how many tabs are running and thus help with troubleshooting. Having said that, if this might taxing then make optional.

Finally, thank you, THANK YOU for your hard work <3