Can't confirm:

System information

4.0-dev
Sample Data
macOS Sierra, 10.12.6
Firefox 57 (64-bit)

MAMP 4.2

• Apache-Server
• PHP 7.0.22
• MySQLi 5.6.35

MY Firefox is 57.0.4
PHP 7.1.8

I'm also on 57.0.4.

Are you using last available 4.0 dev?

I'm on Custom Url https://update.joomla.org/core/nightlies/next_major_list.xml and updated before (got "You already have the latest Joomla version, 4.0.0-dev.").

Here, clean install of last real branch (not nightly) gives

Something wrong in the kingdom of Denmark

As far as i know nightly are updated Branch (These packages were last built: Friday, 19 January 2018 02:00:26 UTC).

Console errors:

Also
[19-Jan-2018 08:30:20 Europe/Berlin] PHP Notice: Undefined offset: 0 in /Applications/MAMP/htdocs/newfolder/joomla40/layouts/joomla/form/field/radio.php on line 71
[19-Jan-2018 08:30:20 Europe/Berlin] PHP Notice: Trying to get property of non-object in /Applications/MAMP/htdocs/newfolder/joomla40/layouts/joomla/form/field/radio.php on line 71
[19-Jan-2018 08:30:20 Europe/Berlin] PHP Notice: Undefined offset: 1 in /Applications/MAMP/htdocs/newfolder/joomla40/layouts/joomla/form/field/radio.php on line 72
[19-Jan-2018 08:30:20 Europe/Berlin] PHP Notice: Trying to get property of non-object in /Applications/MAMP/htdocs/newfolder/joomla40/layouts/joomla/form/field/radio.php on line 72

@franz-wohlkoenig
Please do a clean install and not an update.

Use
https://developer.joomla.org/nightlies/Joomla_4.0.0-dev-Development-Full_Package.zip

I will, but isn't the nightly Build to not must start by Zero?

Normally, yes. But there could be a problem when updating.
Specially
[4.0] Remove grunt in favour of native node build scripts (#18446)
is removing some files and the updater may not have deleted these.

Issue confirmed on Firefox.

As you also have remarked, and that is not only on Firefox, the Statistics panel is displaying over the body and has no background.

Correct, the Statistic Panel-Issue is also on Safari.

Expected! The code in the other repo custom elements needs update

@dgt41
Understand. When will it be updated as we just can't test on Firefox?

I'll send a PR later on

Set to "closed" on behalf of @franz-wohlkoenig by The JTracker Application at issues.joomla.org/joomla-cms/19403

closed as having Pull Request #19407