Thank you for the clarification. 
What disadvantages does it have, if I take a stable build in which the commits are not included?
Is there anything that I can check, that I do not get a very unstable build?
It seems to be daily snapshots, what is the likelihood that I could brick the device. 
I'm using ArchLinux and LineageOS, but I'm still new to OpenWRT and how it works. Please do not mind, if I ask so many questions to make sure.
I'm actually not sure about that, seeing as I never used a release build. Judging from the commits that went into master you would miss out on half of the flash memory space, generally better support, and of course the newer kernel version 4.14 (18.06 is still on 4.9 AFAIK). Generally speaking, right now the release build is about 9 months behind on all improvements that since went into master.
Of course there's always the possibility that something broke in the snapshot you get on a particular day. I have yet to experience that, though.
With this particular device it's practically zero. Since you're using the bootloader to FTP in new images, as long as you don't overwrite the bootloader (and you'd have to do rather silly things to do that) you can repeat that process any time.
Many Thanks.
I still have a general question.
In the following blog post stands:
This means that the luci web interface is not pre-installed and you should only install new packages within the first few days after flashing (so you don’t pick up newer packages incompatible with your firmware).
Does that mean I have to install a web interface later manually? This is quite unfortunate if the device is not configured with the internet ...
https://blog.michael.kuron-germany.de/2018/12/openwrt-on-avm-fritzbox-3370/
Correct, alternatives include waiting for 19.03.0, using imagebuilder with snapshots now and including luci into the image you're going to flash or building from source.
Thanks for the clarification.
When is 19.03 expected to appear?
Do I have to install something besides luci gui which is necessary for normal router operation?
Can I connect the 3370 in the short term behind a 6490 Cable only to install luci or gives that problems due to cascaded router? Because I'm not able to set anything as newbie.
cascaded router shouldn't be a problem for installing packages.
At this point, "19.03" is still conjecture, I'm not aware of any announcements (CMIIW).
I wanted to suggest that, but then I remembered that the 3370 does not have a dedicated WAN port that would use DHCP out of the box, and all the ethernet ports are assigned to LAN.
Since, if you're using snapshots, you will have to use the command shell to install the LuCI package anyway, I would suggest you get at least very superficially familiar with the command shell and edit the /etc/config/network file to set up the DSL connection. It's really not hard, and a good skill to have anyway.
By using Archlinux on my private computer, I feel at home in the shell.
Where can I get info which packages are in the stable, but not in the snapshot (like luci)?
Is there somewhere a guide / tutorial for a snapshot installation? On the OpenWrt website is indeed assumed a stable installation.
It's pretty much that, LuCI will be missing. Everything else that's basic to a release build for the device is also in snapshots.
curl -s https://downloads.openwrt.org/releases/18.06.2/targets/lantiq/xrx200/config.seed | grep ^CONFIG_PACKAGE
Just to be clear on this, luci (and with that the full web interface, along its optional plugins) is available for manual installation for snapshots as well - it's 'just' not preinstalled and part of the downloadable snapshot images.
That is correct.
@Phoen7x
isn't a Telekom Speedport Entry 2 or Vigor 165 in Bridgemode + a ath9k based Router a better choice?
Those are an option, if you're looking for a two-device solution (dedicated VDSL modem and dedicated router) - which does have its merits (a faster router than lantiq xrx200 can offer). But unless you need supervectoring (profile 35b) within the next ~6-12 months, the options mentioned above will be much cheaper (20-30 EUR, below 20 EUR is possible (5-10 EUR for an OpenWrt compatible lantiq device without good wlan --> pure modem) if you're patient, versus >120 EUR for the vigor 165, ~100 EUR for the non-supervectoring vigor 130) - and you can also run free software (with the binary xDSL firmware blob) on the modem itself (or all in one device).
Thanks for the hint, but I wanted to have everything in one device.
In addition, the Fritzbox arrived today.
Maybe I'm too stupid for the commands but when I enter in the terminal behind the ftp>:
quote SETENV linux_fs_start 0 ill get the error
---> SETENV linux_fs_start 0
500 'SETENV linux_fs_start 0': command not understood.
If I also leave the quote off, ill get ?Invalid command
This is a bit confusing, because you are only asked for username and password at the beginning and there must also omit the quote USER.
What FTP client are you using? Basic linux commandline ftp would be preferable since it doesn't pre-process your commands.
Edit: Looks like you're not on the bootloader FTP, but on the NAS FTP after the box has fully booted. You need to FTP in directly after power on, the bootloader will accept FTP for a few seconds.
I'm running Parabola GNU/Linux (ArchLinux Fork) with the xfce-desktop and the xfce4-terminal.
Is there any additional package needed for ftp?
Maybe it makes a difference at which time of the boot process I enter the command ftp 192.168.178.1? Because I'm asked directly for a username and not as in the wiki only ftp>.
Entering quote USER adam2 makes no sense.
Yes, it does. Directly after you power on the device, ethernet will come on for a few seconds, and that's when you quickly FTP in.
Thats very tricky... 
I've tried it at different times, and also interrupt boot by pressing a key but always the error message:
ftp: connect to address 192.168.178.1: The network ist unreachable
ftp: no response from host
The first second Power and Info-lights bright red. Then Info-light flashes three times, then Power-light flashes.
It's important that you set your computer's adapter to a static IP 192.168.178.20 with subnet 255.255.255.0, the bootloader and its FTP server will not give you DHCP.