That is a default Google popup there is no stopping that. If you use Google expect to see that popup. I've see this countless of times but if you close the it the first time the next time you open the Browser it shouldn't show up unless you clear cookies then it might show backup. Otherwise you should contact Google support and submit a issue to them about it.

Firefox doesn't have a setting to suppress that dialog, but there is a well-known add-on named uBlock Origin that can do it (it also suppresses a lot of annoying ads). See:

• Add-on: https://addons.mozilla.org/firefox/addon/ublock-origin/
• Specific setting: https://www.reddit.com/r/firefox/comments/yoo6oa/how_to_disable_sign_in_with_google/