Firefox doesn't have a "block site" setting. But it does block specific features, such as popup windows, on a site-by-site basis. Perhaps you could try this:

Open the Options page as follows:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the left column, click Privacy & Security.

Then scroll down to the Permissions section and check the Settings and Exceptions buttons to see whether you find the site blocked under any of those. You should be able to remove that block there if you find it (highlight and click Remove; do not use Remove All).

Thank you for sharing your add-ons list. Your extension named "Email" looks a bit suspicious. Could you try disabling it? You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions.

Then you can review the list on the right side. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why should be disabled or removed.

Any improvement?

Thanks for your help...it did not solve the problem, unfortunately. Perhaps it was Windows, not Firefox. I get these notices from time to time, and never had such a problem. I'll pay more attention to the details the next time I see it. But I didn't want to let your rapid response go unacknowledged.