There are userstyles meant for Elementary, two of them at the moment (with tabs on top or not). For that you need the Stylish addon.
A thing about these two userstyles is that, unlike themes, they can be used on top of the custom (complete) theme: they will make the theme more "elementarish" but will look different depending on that theme, that is: in order for the end result to be close to a full integration with the elementary look, pick a complete theme that looks better with the styles. You can use the default theme for that but also look for a different one that may seem more close to elementary OS.
I have tested with Firefox v. 50.
A few examples of the tabs-on-top userstyle with different themes:
With the default theme:

With FXOpera:

Mx3:

Maybe better, Mx4:

Or, maybe even close to our purpose, the same theme with the other style:

Take care not to enable both userstyles at the same time.
There is also the Linux Gtk+Native style for OS integration. It replaces the complete theme (unlike the two above) and looks like so:
