You need to backup the logins.json and key3.db files. The latter stores the encryption key (master password) and is needed to access the passwords stored in the logins.json file. If the MP no longer works then there is a problem with either of the two files.

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

If you use the Time Machine then try to restore the logins.json and key3.db files from that backup.

• http://www.apple.com/support/timemachine/

Thank you! Now I know I have backups of the key files and will feel confident following the directions to reset my master password.

I spoke too soon. I backed up the files as recommended. Then I followed the steps to reset my master password. Then I copied the saved files back into the appropriate profiles folders. (There are two: "default" and one with my name.) Then I quit Firefox and rebooted my computer - and now my new master password isn't being accepted. (Neither is my original master password.) Help?

I should add that I recently updated to OSX Yosemite, and I think the problem first occurred after I installed the compatible Java Runtime Environment.

And the window that keeps popping up says: "Password Required" under which it says "Please enter the master password for the Software Security Device."

The MP information is stored in the key3.db file. So if you replace this file with another copy then the MP would change as well.

Another way to test the MP is by logging in to the Software Security Device in the Certificate Manager window.

• Firefox > Preferences > Advanced > Encryption: Certificates > Security Devices: Software Security Device: Log In button

I understand from your comment that I can't reinstall the key3.db file after I create a new password. I tried copying the saved logins.json back to my profile (both the default and the one with my name), but that didn't restore my passwords to Firefox. Is there a way to restore my passwords, or do I need to resave them all now that I've had to create a new master password? When I copied my backed up key3.db file back to my profile, it caused the master password problem that I started with. Thanks!

If the logins.json or signons.sqlite and key3.db files are matching and do not have errors then Firefox will ask for the MP and show the names and passwords in the Password Manager. If that isn't the case then best is to start with fresh files and let Firefox create new key3.db and logins.json files. In that case you will have to resave all the passwords.

You can try these commands in the Browser Console (Firefox/Tools > Web Developer):

• Services.logins.getAllLogins().length => Prints number of decryptable logins only
• Services.logins.countLogins("", "", "") => Prints number of all logins including non-decryptable logins

Toggle the devtools.chrome.enabled pref on the about:config page to true to enable the command line in the Browser console

• https://developer.mozilla.org/Tools/Browser_Console#Browser_Console_command_line