You should never connect your computer to another account as that will merge your personal data to that account, but her data should be on your computer unless you choose to remove her data from your device when you disconnected her account and reconnected to your account. If there in only your data on the Sync server and her data is gone then it sounds that Sync saw this as an initial sync and it might not be possible to recover her data.

You can install the About Sync extension in Firefox 72 and newer on desktop to check what data is stored on the Sync server via the about:sync page provided by this extension.

• About Sync https://addons.mozilla.org/firefox/addon/about-sync/

Close and restart Firefox to access the about:sync page added by this extension (see the about:about page for possible about pages).

Thanks for the response. I logged out from my account before logging into hers - so surprised data was merged. Weirdly enough it didn't merge everything - I have some logins, but not others.

If you use one account aka Windows account and one browser data - one will replace the other. If you want separate Firefox on yours and hers you need to make separate Windows accounts to separate each account from changing the other. This will leave hers and yours separate account with Firefox on two accounts. If you don't do this her Firefox will be replaced with yours and vise versa.