You can throw me into the Firefox/Vista/Avast pot... Except for me it happened right at the worst time, as my 1 year old daughter knocked laptop off couch, had to replace the hard drive and reinstall Vista...
Laptop is a HP/Compaq Presario CQ60-210US...
Here is the ridiculous path I took to finally realizing it was Avast....
After the fall the machine had several bad sectors (probably due to a head crash), and most of the data was recovered but the drive was toast... So when I ordered a new hard drive I also ordered a 4GB RAM kit, I figured I might as well upgrade from the 2GB kit that was in there.
So I get the new drive and RAM, install both, reinstall Vista 64-bit (ohh that was fun, I'll spare you the details) and everything is fine, I install all the applications I use, everything seems to be fine.
When I first encountered the hard locking I instantly worried that there had been hardware damage from the fall other than the hard drive, so I ordered a new motherboard, and a new screen since the old one had vertical lines in the middle due to wear (unrelated to the fall)...
The screen arrived first, worked perfectly fine, but I was still getting random hard locks, always when using Firefox on certain websites... I was starting to suspect the 4GB Ram kit I had ordered, as the hanging started right around the time I installed them (and the new drive).
I decided to swap for the old 2GB RAM kit, and I thought I found the issue, only for the system to hang unexpectedly again... Doh! At this point I am almost certain it's the motherboard and I had not received the new one yet, so I take out the old one and reflow the Southbridge chip which has known issues in these models... Put everything back together... Still locking up in firefox...
Finally the new motherboard arrives, I put it in, and.... Locks up in Firefox, so it wasn't the motherboard... Not long after ordering the Motherboard and just to be sure, I ordered a new Turion 64 X2 RM-74 which is cheap on Ebay these days and much faster than the QL-62 already in the laptop, and which allows me to use the new 4GB RAM kit at full 800Mhz speed, in addition to a 200Mhz faster clock on each core, all in the same 35Watt TDP.
New CPU arrives, and I am pretty certain the CPU is the problem, so I pop the new one in, boot her up and open Firefox... No hangs! I DID IT! *wait for it*... Go to Wunderground, check the weather and, DOH! She freezes up solider than a turd in February....
At this point I have changed EVERY major piece of hardware, new screen, new RAM, new Harddrive, new Motherboard, new CPU... NOW I am getting mad, it HAS to be Software at this point...
I reinstall EVERY DRIVER, do a chkdsk with surface scan, sfc /scannow, reinstall Java, Flash, Firefox... I test in IE, Safe Mode, no crashes, has to be Firefox... I try Firefox in it's own safemode, no dice, still locks up... The only thing left to do is test Firefox with Avast turned off...
SUCCESS! No more hanging, not after hours or days... It must be Avast, so I check for an Update, install the "New 2016 Avast", and re-enable all shields, so far so good!
Gee Thanks Avast, perfect timing with the buggy build... I replaced every single piece of hardware in my Laptop, and it was Avast the entire fkn time!