You can disconnect a device from the following control panel, but if you saved your login on the other machine, you may need to change your password. Also, this doesn't clear your data from the other machine.

https://accounts.firefox.com/settings/clients

I want it to delete the computer's access permanently, because right now, I can log in without an email confirmation.

ac1234 said

I want it to delete the computer's access permanently, because right now, I can log in without an email confirmation.

You can log in to your Firefox account using your username and password, without a second method of authentication. But how could someone else at the library log into your Firefox account without knowing the correct password?

I think you can prevent that Firefox from connecting to your Sync account using the steps I mentioned, but you might also want to clean your data from that Firefox. Possibly they have the computers set to clean themselves daily or weekly, in which case your data will be removed automatically. Worth checking with the library staff, I think.

See also:

• https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information