We probably can't help with problems with your Facebook account. But you could consider checking your Mac and all other devices on which you use Facebook for malware and bad add-ons which might be a vector for stealing your account information.

To check Firefox's Add-ons page, you can use either:

• Command+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. Set anything you do not use to "Never Activate".

In the left column, click Extensions. Set anything you do not use to Disabled, or Remove it.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

For the rest of the system, you could start with this scanning/cleaning tool: https://www.malwarebytes.org/antimalware/mac/ (free for home use)

http://www.clamxav.com/ free malware scanner for Mac OS X https://discussions.apple.com/docs/DOC-3291

You can also try https://www.malwarebytes.org/antimalware/mac/index.html