Do you recognize all of the devices connected to your Sync account? You can go to the following page and click "Show" in the Devices section to check for anything unauthorized:

https://accounts.firefox.com/settings

The following article has steps to lock out unauthorized connections, but any Firefox that has been connected up to now will still have your previous set of passwords, so you may need to change your saved logins for important sites ASAP.

Suspicious activity on my Firefox Account

Start with any email accounts you use for account recovery or password changes if you saved their logins in Firefox -- you absolutely cannot afford to lose control of those email accounts.

HOWEVER, make sure you enter the changes on a clean system in case you are infected with a keystroke monitor or other malware that will capture your new logins. Our support article has cleaning programs that may help:

Troubleshoot Firefox issues caused by malware

By the way, if you let anyone else use your computer(s), check with them first. They might have overwritten your login for something with theirs without thinking.

If you can't find the source of the problem, change the sync password. This will remove all data from the server. Then give the new password only to your systems.

You should also change all passwords for everything.