You can also remove that unnecessarily large sidebar header that Firefox insists on not letting users disable via normal means with userchrome mods. Some such setups can be seen at https://firefoxcss-store.github.io/.

"tab bar in titlebar" has been the Firefox default for a number of years at this point. If Linux, your distro/packager of choice may have overridden that. The only true exception to this is tile based WMs (which don't have title bars) but in that case it's still above by default.

You are correct. I have been using sidebar tabs for probably a decade now, so I had completely forgotten about the redesign.