Firefox doesn't use the session's theme

[Ads20000]

Firefox doesn't use Adwaita's theme for it. See the screenshot.

I don't use vanilla GNOME on other distributions so I'm not entirely sure what the correct behaviour is but pretty sure Firefox is not supposed to be using the Yaru(?) theme? I have the communitheme snap (0.1 (1171)) installed.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gnome-session 3.28.1-0ubuntu3
ProcVersionSignature: Ubuntu 4.15.0-34.37-generic 4.15.18
Uname: Linux 4.15.0-34-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.3
Architecture: amd64
CurrentDesktop: GNOME
Date: Mon Sep 17 11:17:45 2018
InstallationDate: Installed on 2017-08-03 (409 days ago)
InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
SourcePackage: gnome-session
UpgradeStatus: Upgraded to bionic on 2018-04-22 (148 days ago)