https://support.mozilla.org/en-US/questions/1102244

Read above and see IF it helps; My ave. expertise is confused by it but it looks promising

You should never connect to Sync on a device that isn't your own especially if that device is also connected to another Sync account. When you connect a device to your Sync account then your personal data stored on the Sync server is downloaded to that device and data on that device is uploaded to your Sync account (i.e. it merges in both directions). If you disconnect and do not remove your downloaded personal data before connecting this device to that other Sync account then your personal data that was merged and is uploaded to that other Sync account and thus present on that other Sync account as well. You will have to clean this up on both devices.

Didn't you get a warning that this device was already connected to a Sync account ?

Hi, thanks for responding. I got no such warning when I signed in. I understand that my information was synced with Sync; my question is what can I do about it now? How do I delete it from the system, "clean it up" as you say? Thanks!

If your personal data is already synced to the other Sync account then you will have to cleanup that other device manually. Otherwise you could have tried to remove all data on that device (i.e. backup and cleanup the Firefox profile) and download the old data by connecting to the old Sync account. You will have to do the same with you own device.

I'm sorry, I'm not sure I'm following you. Could you explain what exactly I need to do on each computer? I don't know what you mean by "clean up manually" or "backup and cleanup the Firefox profile." I have gone through and deleted the relevant sites and logins from the computers history and cookies and they are still showing up.

Here's how I diagrammed the problem (which may be too convoluted, but here goes): So I have my personal computer A on which I use my personal Firefox account P. I also have a shared computer S, on which I was originally not using a Firefox account, then signed in as P, saw what it imported, signed out, then made a new Firefox account X. I also logged in with X on another computer S2. When I saw X had all of P's info, I deleted X. I deleted absolutely all the history and data on S and S2, and uninstalled/reinstalled Firefox on both. Now I am not signed in, but still have lingering P information on S and S2. Weirdly, that information isn't coming up in the search bar on my personal computer A, but it was earlier on S and S2.

FWIW my personal computer is a Windows machine and S and S2 are Macs.

I appreciate your help; thanks for responding.

Thank you to everyone who replied! I appreciate your help. I wanted to let you know that I figured something out that I think did the trick. I uninstalled Firefox and downloaded an older version of Firefox. Then I turned off my wifi and launched the old Firefox. I got a warning that it was an older version of Firefox and I'd need to create a new "profile." I clicked okay and it launched... and no more private info! All the information was gone. Did the same thing when I downloaded and reinstalled the newest Firefox. So if anyone else runs into this problem, try reinstalling while offline.

Note that this still leaves old profiles on the hard drive.

Firefox uses two locations for the Firefox profile folder. One location for your personal data (e.g. bookmarks) and another location for temporary files like the disk cache.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Location used for the main profile that keeps your personal data (Root Directory on about:profiles).

• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>

Location used for the disk cache and other temporary files (Local Directory on about:profiles).

• C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>