Active tab in dark mode is hard to identify.
I do not want to install add-on. in order to be able to make Active tab different color or border or any different than other tabs Why is it hard to make the Active tab same as in Edge browser?
Each browser uses their own color theme and Firefox looks different than a browser like Edge. Without using an Add-on your only option would be to use CSS code in userChrome.css.
You can use code in userChrome.css to override changes in Firefox 89+ with the new Proton design.
You can look at this extension:
You can also create your own theme: