The Firefox Browser provides useful options for privacy and security, but there is no option to specifically block problematic iframe dialogs such as the Google sign-in popup frequently observed on many websites.
If the recurring Google sign-in dialog detracts from your browsing experience, the Firefox 'Ublock Origin' extension provides many useful options for content blocking and is easily configured to prevent most instances of the Google sign-in popup.
1 - Install the Ublock Origin extension
2 - Go to Ublock Origin Preferences -> My filters (tab)
3 - Insert the following line:
Code: Select all
||accounts.google.com/gsi/*$xhr,script,3p
4 - Click Apply changes
See images below: