I don't know why google does it, but try to delete google cookies.

We have resolved an issue with Google prompting users to ‘clear cookies and site data’ when logging in. A fix has been published, via Firefox remote settings that does not require a Firefox upgrade. Users will receive the update over the next 6hrs, if you would like to update immediately simply restart Firefox.

Note: The issue was related to CHIPS being enabled in Fx131, this feature has been temporarily disabled while we resolve the issues. Users who have disabled 'Allow Firefox to install and run studies' in Privacy & Security settings will need to enable to receive the update.