Icons in folder pane don't scale for hidpi, breaks rendering

[ts]

Ever since the update to 78.7.1 (64-bit) Thunderbird attempts at showing icons next to the folders in the folder pane. Hidpi settings cause the font to scale, but the icons don't scale (at the same level? at all?). This entirely breaks the rendering of the folder pane on high resolution displays with downscaled fonts.

Screenshot attached.

$ lsb_release -rd
Description: Ubuntu 20.04.2 LTS
Release: 20.04

$ apt-cache policy thunderbird
thunderbird:
  Installed: 1:78.7.1+build1-0ubuntu0.20.04.1
  Candidate: 1:78.7.1+build1-0ubuntu0.20.04.1
  Version table:
 *** 1:78.7.1+build1-0ubuntu0.20.04.1 500
        500 http://de.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1:68.10.0+build1-0ubuntu0.20.04.1 500
        500 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages
     1:68.7.0+build1-0ubuntu2 500
        500 http://de.archive.ubuntu.com/ubuntu focal/main amd64 Packages

I expect Thunderbird to show a neat folder pane

It doesn't, rendering is broken due to failed scaling of the icons

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: thunderbird 1:78.7.1+build1-0ubuntu0.20.04.1
ProcVersionSignature: Ubuntu 5.4.0-66.74-generic 5.4.86
Uname: Linux 5.4.0-66-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
BuildID: 20210203182138
CasperMD5CheckResult: skip
CurrentDesktop: MATE
Date: Tue Mar 9 07:58:06 2021
InstallationDate: Installed on 2020-03-26 (347 days ago)
InstallationMedia: Ubuntu-MATE 20.04 LTS "Focal Fossa" - Alpha amd64 (20200322)
SourcePackage: thunderbird
UpgradeStatus: No upgrade log present (probably fresh install)

[ts (toto-23)]

OK, re-enabling userChrome.css according to

http://forums.mozillazine.org/viewtopic.php?f=30&t=3064381&start=6

and creating a userChrome.css as follows:

/* Thunderbird userChrome.css */

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

#folderTree > treechildren::-moz-tree-image {
  width: 10px !important;
  height: 10px !important;
}

#folderTree > treechildren::-moz-tree-cell-text {
  padding-bottom: 1px !important;
}

provides a temporary solution!