Kamikaze
It's not really answering  my question. It's talking about the warning you not to do the update because its a Trojan, not what happens if you do do the update. What happens then.
 
 

 
If it is downloaded it seems to be a number of things, A Cryptolocker, keystroke logger, browser hijack,,
 
 
By default, Firefox is set to automatically update itself but you can always do a manual update. Here's how:

1. Click the menu button , click help and select About Firefox.
2. The About Firefox window will open and Firefox will begin checking for updates and downloading them automatically.

So it seems your update may be safe but AFA your display issues did windows do an update also? you may have driver issues
 
If your still concerned about infection did you run your AV and Spybot and Malwarebytes?

I'm starting to think my Firefox is a legit update gone wrong. Installed Chrome and evreything seems fine, though I don't like it. It's my work mini laptop, not my music laptop. Malwarebytes and AVG have both come back clean. So I think I need to try a clean install of Fire fox in a few days, when I get a little space to do so.

Kamikaze
I'm starting to think my Firefox is a legit update gone wrong. Installed Chrome and evreything seems fine, though I don't like it. It's my work mini laptop, not my music laptop. Malwarebytes and AVG have both come back clean. So I think I need to try a clean install of Fire fox in a few days, when I get a little space to do so.

Glad your scans came back clean.
Maybe post your screenshot over at the FF forum and see if anybody else had the same experience?
They seem to be a helpful lot over there

Kamikaze
It's not really answering  my question. It's talking about the warning you not to do the update because its a Trojan, not what happens if you do do the update. What happens then.
 
I saw Firefox was updating itself, at the weekend and since then, it's been a bit funky with lines of a screen stretching across pages. Maybe it was a proper update, but is buggy, maybe it was trojan and the symptoms. Perfect timing, This is what is happening.

 
 

Looks like you've found the bar code add-on!