I assume that Firefox OS should display the mobile frontend instead? Would be interesting to know which user agent string is sent, and this definitely needs retesting as this was three months ago.

1. MobileFrontend is, for the time being, an extension. The bug described here is an obvious issue with MediaWiki core. The site should remain usable without any extensions, and that's what it will be if this bug is fixed. Zooming in allows to read the (content, not sidebar) text just fine.
2. User-Agent was posted on the initial report
3. Even though the screenshot was taken in late June, the bug still persists, at least with MediaWiki 1.21.2, but if you know of a non-MobileFrontend wiki running bleeding edge code, let me know and I'll happily test to see how my Keon renders that site's Monobook.

Created attachment 14273
Firefox OS Simulator screenshot

Installed https://addons.mozilla.org/en-US/firefox/addon/firefox-os-simulator/ and went to http://www.shoutwiki.com/wiki/Main_Page .
Cannot reproduce the problem.

Attached:

As Jack Phoenix indicated, the simulator isn't accurate to what the software on the physical device renders.

Andre, thank you for taking the time to try to reproduce this bug. Unfortunately the Firefox OS simulator is somewhat of a joke -- and a bad one, since it doesn't make anyone laugh. The simulator produces essentially different results than real hardware, which isn't very obvious if you don't have access to a real Firefox OS device, sadly.

Since filing this bug, I've updated my Keon's Firefox OS to (software) version 1.1.1.0-GP, but the bug still persists. It can also be reproduced with (certain?) other sites, such as WikiIndex (http://wikiindex.org/?useskin=monobook), and other Firefox OS devices, such as the ZTE Open.

Just for this bug, I tried out ShoutWiki's and WikiIndex's main pages in my ZTE Open, which has the following specs, and the bug is happening on both sites:

ZTE Open
OPEN_EU_DEV_FFOS_V1.0.0B02
OS version 1.0.1.0
Platform version 18.0
Build identifier 20130906173233

Git commit info 2013-09-06 09:20:54
cb3f650b57e63566872343d023b4715...

I tried this weekend (at the Firefox stand at FOSDEM conference) on a Firefox OS device with OS 1.3.
I went to www.shoutwiki.com and did not see any issues with the sidebar.