I have a really strange behavior with highchart (via rails plugin): The graph display without the graphlines but the data exists because when point to the graph it's show the little box with the data.
http://www.alderia.com/highchart1.png
When I open firebug, the lines in the graph appear...
http://www.alderia.com/highchart2.png
The thing is that the same thing happens with Chromium... so it's something not specific to firefox.