You can remove all data stored in Firefox for 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 for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for 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 for that website will be saved once again.

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

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

You can do a malware scan just to be sure.

• https://support.mozilla.org/de/kb/troubleshoot-firefox-issues-caused-malware

You can check for issues with location storage (indexedDB).

• https://firefox-storage-test.glitch.me/

This could be a problem with local storage and possibly running this command in the Web Console to clear storage as posted in bug 1569296 might fix it.

• localStorage.clear()

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/Tools/Web_Console

• Bug 1569296 - Facebook pages loading blank

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)