Update (22nd September, 2014): Ever since I upgraded above a certain developer preview version of OS X Yosemite (DP4 or thereabouts) a couple of months ago, the problem described below has not recurred.
I've got a mid-2013 13" MacBook Air (Model Identifier: MacBookAir6,2) currently running Mavericks 10.9.2. that doesn't have any problems except (a potentially major) one: Sometimes when I'm playing a Flash video on Firefox (currently version 28.0), suddenly the screen will freeze completely - by "completely" I mean even the mouse pointer won't move. Usually the audio plays for maybe a second and two after the screen freezes before it stops too. (I'm guessing that just might be some kind of audio buffering in action.) The system goes completely unresponsive. I have to force a reboot by holding down the power key for several seconds.
This happens intermittently but frequently enough to be a cause of concern. I guess it must have happened around 3-4 times in the last month. The latest incident was last night.
I have not experienced this problem except when I'm viewing a video online. I'm running the latest version of Adobe Flash Player (which, btw, has annoyingly frequent updates) and Firefox 28.0. but it has happened with previous versions of both. I mostly view only streaming videos on this laptop - only rarely do I play a downloaded video file with a media player app, so in that respect it's hard to say whether the problem is Firefox/Flash specific or more general. But I'm pretty sure it's never happened when I wasn't viewing a video. I believe it also happened on lower versions of Mavericks.
I checked the system log in the Console app, and have not found any suspicious looking messages that could indicate what the problem could be, which makes me think it's something that happens all of a sudden - making it likely to be a hardware problem with the video subsystem. But I'm just guessing here. (There are also no log messages generated after the time of the freeze until I force a reboot, which would indicate that the system has in fact completely halted, rather than a case of unresponsive UI.)
I have run "Apple Diagnostics" more than once and it reported no issues.
How can I diagnose this problem? Is there a more severe battery of hardware tests that I can run? Or should I look into using DTrace?