Mac Theme for Firefox
For a long time, there has been a theme for Firefox called the MacOSX Theme, which is used to make Firefox look like macOS. Unfortunately, this add-on has not been maintained as well as it should be, and the theme no longer works on current versions of Firefox.
Don't worry, though. You can download a compatible version here: f1beta.com/macosx_theme_firefox_4-1.8.1-fx.xpi.
Note: This plugin is the work of golf-at and can be found online at Mozilla Add-ons.
I have patched it for users who are having trouble installing it on the latest version of Firefox.
Alternatively, you can still use the theme from the original site by using this compatibility hack: CheckCompatibility, which helps run unsupported plugins.