Firefox closed because it could execute some sort of malicious code or something. I've just installed the latest Flash player, so maybe that's something to do with it. I've run a virus scan with Malwarebytes and it's said nothing was detected. Help?

What was the exact message you got? Was it something like this:

Data Execution Prevention – Microsoft Windows
To help protect your computer, Windows has closed this program.
Name: firefox.exe
Publisher: Mozilla

Data Execution Prevention helps protect against damage from viruses or other threats. Some programs might not run correctly when it is turned on. For an updated version of this program, contact the publisher.

If your security programs are up-to-date and you've run scans and found nothing, it's probably not malware. DEP is designed to prevent certain security exploits, but it can sometimes misidentify what a program is doing as malicious; I've had this error a few times with iTunes. Don't worry about it unless it happens often.

Almost exactly like that, thank you, and this is the first time it's happened as far as I can remember.