There's no easy way to put this, so I'll just be blunt. You will loose all your saved usernames and passwords if you have to reset your master password. This is because anyone who gains access to your computer, locally or remotely, can reset the master password. Allowing recovery of all the sensitive information stored there would be a major security risk and you wouldn't want that info falling in to the wrong hands.

Suggestion: Give yourself a few more days to try and remember your password, otherwise you can reset your master password by following the instructions on the support article linked below.

How to reset your master password: https://support.mozilla.org/en-US/kb/reset-your-master-password-if-you-forgot-it

If you never have set the master password yourself then you can try to rename/remove the secmode.db file in the Firefox profile folder.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

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