Just like you said, if you have taken steps to ensure that the host OS is not compromised, it's highly unlikely that it was read, but there's always a chance that some of the unencrypted database was written to disk and may remain even after you set up a new master password & key. It depends on things outside of your control, such as the HDD or SSD's disk controller. You could mitigate that somewhat by using a utility to wipe free space on your drive.
In the longer term, if you consider your passwords to be this valuable, you might want to consider using a more sophisticated method for protecting them, such as KeePass 2. I use it myself to keep dozens of credentials at work and at home, and I believe everything is stored with AES256 or higher by default. You can then use KeeFox to integrate into Firefox (the dev is working on porting it to the new WebExtensions framework too).