As you can see above, the close button is on the right Windows-style. But I want to make it fit with the elementary look. I think this requires messing around with Firefox's
userChrome.css but I have no idea what to do from that point.
There is a theme available which makes Firefox look more native to elementary OS:
My brand new elementaryOS install has the close button on the left hand side. It looks like you have client side decorations enabled which saves vertical real estate by putting UI elements in the title bar.
To disable this, visit
about:config in Firefox and set the preference
false and restart Firefox.
You can use the elementary tweaks.
Open a terminal, then write and execute (by pressing ENTER) this commands, one at a time:
sudo apt install software-properties-common
will ask your password
sudo add-apt-repository ppa:philip.scott/elementary-tweaks
will ask you press ENTER to confirm the installation of the PPA
(newer version of add-apt-repository runs apt update after automatically. In case it didn't, just run sudo apt update yourself)
sudo apt install elementary-tweaks
This was answered before where it belongs
Look you can use someone's work, as a start point, for an old version of Firefox