Helping people with computers... one answer at a time.
Once your machine is infected, it's impossible to know that it's been completely cleaned. I'll look at the options, both ideal and practical.
My Windows XP PC was infected by some viruses, which had changed some registry settings before they were removed. I noticed the infection after I found the malware called "SmartProtection 2012" was unexpectedly installed in my PC. After the virus removal, I now have both McAfee and Malwarebytes up to date and run regular full system scans to check if there is still something lurking around. Nothing suspicious is reported. But two problems remain:
(1)After this, the internet browsers (both I.E & Mozilla) always crash unexpectedly, especially when downloading a file (even just a small 20MB file).
(2)My Windows Security Center has been stopped and there is no way I can find to turn it back on.
You're not going to like my answer.
And, unfortunately, it's an answer that I end up giving somewhat often, and in fact, I've even written up before.
I'll give you a thought or two on perhaps dealing with at least one of the issues that you're facing.
You'll quickly understand why malware infections are best avoided completely rather than trying to clean up after them.
Once you've determined that your machine has been infected, the simplest solution by far is to restore your machine to the most recent backup taken immediately prior to the infection.
Poof! Infection gone. Completely.
Pretty cool, huh?
Given how easy and complete that solution is, it's very disheartening to hear how many people don't have that as an option.
Because they haven't been backing up their machine at all.
At the opposite end of the spectrum is the only other way to guarantee that the malware has been completely removed and that all lingering traces are gone as well.
Backup your data.
Reinstall Windows and all your applications.
Restore your data.
I'm tempted to add a fourth step: Start backing up.
The problem is something I've mentioned several times before:
Once it's infected, it's no longer your machine.
Even if you think you've successfully removed the malware, you have no guarantee - none - that there's not still something left over. Perhaps it's malware still quietly doing whatever malware does. Perhaps it's just a missing file that you won't realize until you need it some weeks from now.
Perhaps there's nothing wrong at all.
The problem is you just don't know.
The only way to know is to wipe the slate clean and start over.
The problem is that no one wants to do that. They'd rather live with the risk of still being infected.
Because, of course, it couldn't happen to them.
Even though it already did.
What we're left with is what you're asking for: fixing the symptoms you notice.
For Firefox, I'd uninstall it and reinstall it.
I honestly can't tell if you that will in fact resolve the issue.
But short of the other solutions that I've mentioned above, it's your next best bet.
I'm not trying to be a smart-ass, but there's only one "real" solution.
Don't get infected in the first place.
As you can hopefully see by now, the cost of getting a malware infection can be very high, particularly when you factor into account properly and completely recovering from it.
Staying safe to begin with is much more effective.