No longer able to access Gmail in Firefox

When you're on the error page, could you please click the shield icon next to the address bar and "Clear cookies and site data" and try again? (Perhaps even lowering the tracking protections in that shield pane if it makes any difference.)

If you open a new Private Window and try signing in there, is it any different?

Thank you, jbr.

Question: Is there any way to clear cookies and site data for Google only?

I am concerned about having to log into all of the websites I have bookmarked. All the usernames and passwords. I have a password manager but it would still be time consuming.

The word "may" is used. Such as "you may have to log in to all of your websites." In your experience will it wipe out log in information for everything?

Right, absolutely appropriate question to ask in this context!

The shield icon for that one particular site is designed to clear exactly just this relevant cookie partition, so it "should" only remove the google.com — and by "should" here I mean it eventually may clean "less" than you'd like — not "more". So it won't remove any session from unrelated sites, but might still keep some Google–adjacent ones in place.

If you want to have full control over this process, and check for any other relevant domains of theirs, you can actually remove these manually by going into Settings, and searching for "manage browsing data" option that will point you to the Privacy section entry that will open a panel for you, where you can select individual sites, and then "Remove Selected" and confirm with "Save Changes" when you're finished, to have a full control of what data partitions you're removing. The article Manage local site storage settings has the steps and images for your reference.