Upgraded memory and I'm getting errors in Memtest, system unstable

[Exploration]

I upgraded my Acer Aspire One AO722 to 8GB using PNY 1x8GB 1.5v DDR3 1600 module. Official recommendation is 4GB max. This computer has one slot. I went forward with the 8GB upgrade after reading success reports for this specific combination on various websites.

Windows 7 Home Premium 64 bit sees all 8GB and can utilize it, but after the upgrade, Firefox started crashing randomly, but other than that, it appears and works as successful upgrade.

I stuck the original 2GB module back-in and the problem stopped. I let Memtest86 run overnight and it was seeing errors in the higher end area starting at test #5, so set it to repeat test #5 in the higher addresses and I repeatedly get errors. Stick the 2GB that came with the system back in and it'll run all the test all night with no errors. No Firefox crashes.

I constrained the test to the upper range and repeating test 5 just to duplicate the errors in shorter time.

The BIOS does see all 8GB. 8192MB + 256MB video RAM.
It shows the system memory as 7915MB in Memtest, but the upper bound is set to 8432MB.
I am getting a bunch of errors in the 8000MB-8432MB range.
No idea where 7915MB, or 8432MB came from.

So, what is going on here? Is it because I am using 8GB when the recommendation is 4GB or is the new RAM bad?

[David (PassMark)]

I think the most likely problem is that the BIOS not supporting the 8GB of RAM.
I assume there is no firmware upgrade available?

Conclusive test would be to test the 8GB stick in a desktop machine or swap the stick for another one.

[Exploration]

BIOS sees all 8GB.

[David (PassMark)]

Yes, it is a good start that 8GB is reported, but does BIOS correctly map the addresses of all the devices into the 8GB space. The answer to this is pretty much impossible to know, unless it goes wrong. If the manufacturer never tested it, you can't assume it will all be good.