Every time I open an XML file, all I get is blank page instead of tag tree. The file itself is correct and loads okay, I can see it via View Source or in the Firebug. I've tried turning off all my addons and tried running Firefox in safe mode, but the problem was not solved. 

I'm guessing that I've messed up my configuration somehow and Firefox now tries to render XML files as HTML ones. I've tried googling, but with no success. Help, please?

UPD: example file: <http://lj.lain.ru/3/1273657698603.sample.xml>
Also I've noticed that somehow all of the browsers on the machine are now acting the same, so I'm changing the question accordingly