Hello Guix Users, So apparently all that was needed after the steps suggested on the mailing list was a restart of my machine or possibly a re-login. I say that, because today I started my machine and started Icecat and to my surprise the fonts were not all monospace any longer. So it must have been, that some of the things I did worked, but was not picked up by Icecat without restart, or possibly re-login, as it is unusual on GNU/Linux to require a full restart of the machine.
I will try to summarize what I tried or what was suggested to fix it, so maybe if someone finds this in the future it will help them concisely. 1. change preferences inside Icecat for what fonts to use (but in theory this only applies to websites and not to Icecat GUI). 2. create a `chrome` folder in my profile and create a `userChorme.css` inside the folder, containing CSS rules for the Icecat GUI. 3. install fonts via Guix: `font-dejavu`, `font-liberation` 4. run `fc-cache -f` 5. create a `settings.ini` in `~/.config/gtk-3.0/settings.ini` and put in there the following: [Settings] gtk-font-name=DejaVu Sans Book 11 6. install `fontconfig` via Guix and run `fc-cache -f` 7. install `libxft` via Guix 8. install `cairo` via Guix 9. install `pango` via Guix After each of the steps I completely closed IceCat and started it again, which did no change to everything being monospaced. Finally I shutdown my machine yesterday and started it again today and the issue suddenly disappeared. And the warnings also disappeared from command line output when running IceCat from there. Only the following was logged: JavaScript error: jar:file:///gnu/store/rllrc55ygi4yhxnhq8v41ynl6mg5ak56-icecat-60.3.0-gnu1/lib/icecat/omni.ja!/components/XULStore.js, line 65: Error: Can't find profile directory. 1557058011027 addons.webextension.https-everywh...@eff.org WARN Please specify whether you want browser_style or not in your browser_action options. GLib-GIO-Message: 12:06:51.903: Using the 'memory' GSettings backend. Your settings will not be saved or shared with other applications. I do not know what exactly helped, but that is all I can remember trying. I just hope, that when/of I want to change the font of the UI again, it will not take me as long to figure out how again :D But then again how often do I change a GUI font. Thank you all for your help and ideas! Now I have a very usable browser again. If an `strace` output would still be helpful, I could provide that too. Best regards, Zelphir