Under what circumstances would a master password get deleted/unset without the stored passwords getting deleted? This may have immediately followed an update.
I've recently had a paranoia boost from some political activity. My gmail account was attempted to be hacked immediately after I started a petition on change.org. Some two weeks later, I've noticed that my work desktop (running CentOS) had no master password set, but the passwords were still there. I don't think I did that, but there is a good chance I updated CentOS (but not Firefox) over those two weeks.
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The file key3.db in your profile folder is used to encrypt and decrypt your saved passwords. If it's damaged or deleted, your saved passwords would be inaccessible. So there's no chance of the situation you describe being the result of an operating system or Firefox update.
The master password only gets deleted if a user removes it. You uncheck “Use a master password” in Preferences - Security, then you must enter the current master password to confirm the operation.
A variety of Firefox password brute force cracking tools exist. I don't know if anyone can sneak one onto your system or run them remotely though.