I undertand Midori is the default browser but have installed Firefox and would like to make it look nice.
There currently is no theme that really fits elementary OS, but there is a GNOME theme that adds a headerbar to Firefox. You can find it at https://github.com/chpii/Headerbar
It looks like this:
You can install it by downloading these extensions and themes:
- Extension that makes the notifications of Firefox use elementary OS's (optional)
- GNOME theme
- Extension that hides tabs (optional)
- Extension that adds headerbar
To make the headerbar add-on work correctly, open Firefox menu and select Add-ons, then find HTitle preferences and select Hide titlebar: Always. The nice thing is that all the add-ons are modular, so you can just enable and disable add-ons until you like the way it looks. It is also stylish compatible, so you could even add some custom CSS to make it match elementary OS better.
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:
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: