Created attachment 554334 [details]
It currently looks too close to the code imho and the line number text touches the separator line (see screenshot).
Probably a 1px or 2px left/right padding is enough.
see also bug 678995.
This is going to be fixed by bug 692807.
This is now fixed. Bug 692807 landed.
VERIFIED on Build identifier: Mozilla/5.0 (X11; Linux i686; rv:10.0a1) Gecko/20111107 Firefox/10.0a1