Note that if you use dismiss in the '...' menu Firefox adds this URL to a browser.newtabpage.blocked pref that you can find on the about:config page. Delete from History is usually a better choice.

You would have to remove all the history for this website. You can do a search in the sidebar or in the Library for this domain or possibly use "Forget About This Site".

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox from this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

I don't see how to get to "Forget About This Site", I don't have that option listed in the history sidebar. Is there a pic on how to get to it?

You can find "Forget About This Site" in the right-click context menu of a history item.

See e.g.:

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox#w_remove-a-single-website-from-your-history