>> - Ubuntu 16.04 LTS i686
> Are you really using 32bit?

Yes
$ uname -a
Linux x2go-i686-vm 4.4.0-79-generic #100-Ubuntu SMP Wed May 17 19:57:27
UTC 2017 i686 i686 i686 GNU/Linux

I'm running all tests on a amd64 machine too, I get the same results
$ uname -a
Linux x2go-amd64-vm 4.4.0-79-generic #100-Ubuntu SMP Wed May 17 19:58:14
UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

>> - nx set to link=3Dlan, pack=3Dadaptive-9, the rest as default
> Are you also _running_ on a LAN?

Yes, 1Gbps link 1 hop away from the server for testing purposes.
Reduced it to ADSL as a test, artifacts remain.

>> acceleration disabled and rendering by cairo.
> How did you disable hw rendering?

Options -> Advanced -> uncheck "Use hardware acceleration..."

Enabling it doesn't make a difference, the artifacts remain. I'm running
VMs without any GPU support.

For cairo in about:config

gfx.canvas.azure.backends cairo
gfx.content.azure.backends cairo
gfx.xrender.enabled true

> Have you tried the firefox (53.0.2 or newer, not an ESR version) that
> Ubuntu is delivering with the distro? Is it also happening with that
> version?

Yes, current Firefox from Ubuntu repos and the latest 54.0b13 from
Mozilla's build have the same issue.

> I suggest to clear the caches on server and client and then retry with
> a new session. Caches are in ~/.x2go.

No improvement.

> Please provide some screenshots

about:mozilla in Firefox 45.9.0esr
https://abload.de/img/ff-45esr-html1js43.png

about:mozilla in Firefox 52.1.2esr
https://abload.de/img/ff-52esr-htmllns35.png

Differing quality between adjacent elements in Firefox 52.1.2esr
https://abload.de/img/differing-quality-icow4s09.png

Artifacts in the chrome seem to disappear when I resize the window and
it has to be redrawn. They reappear after I hover on them (ie, after
the backgound changes).

> as well as some URLs your are seeing the problem with.

A good universal reference is about:mozilla.

Artifacts in the chrome are noticeable in the "+" to create a new tab,
for example.

> Also I'd like to know the kind of session you are running

Im running Firefox as single application, for instance:
/opt/firefox52esr/firefox -P

> Is your distro uptodate?

Yes, installed from 16.04.2 Ubuntu ISOs, just updated/upgraded.

> In a broken session please post the options file from the server side
> (~/.x2go/C-*/options).

Firefox 45.9.0esr

nx/nx,link=3Dlan,pack=3Dadaptive-9,limit=3D0,root=3D/tmp/.x2go-test/C-tes=
t-50-1497443353_stRfirefox_dp32,cache=3D8M,images=3D32M,type=3Dunix-kde-d=
epth_32,id=3Dtest-50-1497443353_stRfirefox_dp32,cookie=3D11afc8e451487a98=
e241b19d22df6e03,errors=3D/tmp/.x2go-test/C-test-50-1497443353_stRfirefox=
_dp32/session.log,kbtype=3Dnull/null,geometry=3D800x600,resize=3D1,fullsc=
reen=3D0,accept=3Dlocalhost,listen=3D32416,clipboard=3Dboth,client=3Dlinu=
x,menu=3D0,state=3D/home/test/.x2go/C-test-50-1497443353_stRfirefox_dp32/=
state:50

Firefox 52.1.2esr

nx/nx,link=3Dlan,pack=3Dadaptive-9,limit=3D0,root=3D/tmp/.x2go-test/C-tes=
t-50-1497444001_stRfirefox_dp32,cache=3D8M,images=3D32M,type=3Dunix-kde-d=
epth_32,id=3Dtest-50-1497444001_stRfirefox_dp32,cookie=3D531a4eb25bde9f45=
deeded697cc18b62,errors=3D/tmp/.x2go-test/C-test-50-1497444001_stRfirefox=
_dp32/session.log,kbtype=3Dnull/null,geometry=3D800x600,resize=3D1,fullsc=
reen=3D0,accept=3Dlocalhost,listen=3D61183,clipboard=3Dboth,client=3Dlinu=
x,menu=3D0,state=3D/home/test/.x2go/C-test-50-1497444001_stRfirefox_dp32/=
state:50

---

A reproducible scenario:
- Create a new VM. 4 cores, 4GB memory, no video memory, 25GB storage,
1Gbps link
- Install Ubuntu from
http://de.releases.ubuntu.com/16.04.2/ubuntu-16.04.2-desktop-i386.iso
-- Don't install non-free components
- Update/upgrade system
- Install X2Go on server following the instructions in
http://wiki.x2go.org/doku.php/doc:installation:x2goserver
- Install latest X2Go client on Windows 7 machine
- Download Firefox 45esr
http://ftp.mozilla.org/pub/firefox/releases/45.9.0esr/linux-i686/en-GB/fi=
refox-45.9.0esr.tar.bz2
-- Untar to /opt/firefox45esr
- Download Firefox 52esr
http://ftp.mozilla.org/pub/firefox/releases/52.2.0esr/linux-i686/en-GB/fi=
refox-52.2.0esr.tar.bz2
-- Untar to /opt/firefox52esr
- Create a new session in X2Go client: link=3DLAN, pack=3Dadaptive-9,
printing disabled, no port forwarding for audio
- Launch /opt/firefox45esr/firefox -P
-- Create a new profile
-- Set configuration to cairo, enable xrender, disable hardware accelerat=
ion
-- Restart Firefox, check image quality in about:mozilla
- Launch /opt/firefox52esr/firefox -P
-- Create a new profile
-- Set configuration to cairo, enable xrender, disable hardware accelerat=
ion
-- Restart Firefox, check image quality in about:mozilla

The non-free components can be installed later on with the package
ubuntu-restricted-extras, but it doesn't make a difference.