Most annoying change ever. I was to the point of reversing the version because of this. What can you do currently is to disable the Proton theming by entering in “about:config”, then disabling (setting to false) the property “browser.proton.enabled”.
To be honest, I would like the Proton theme, but with the separators back or an option to toggle them. Why in the world they removed them, I can’t understand.