The following address works for me; the new certificate appears to have been generated on May 20th:

https://www.signal-spam.fr/en/

Since it's my first visit, I don't have any cached files that affect how it loads. I don't know whether that is relevant, but you could try clearing the cache (don't clear history, cookies, or other kinds of data, just the cache): How to clear the Firefox cache. If you have a large hard drive, this might take a few minutes.

Any difference?

Try to rename the cert9.db file (cert9OLD.db) and remove the previously used cert8.db file in the Firefox profile folder with Firefox closed to remove intermediate certificates and exceptions that Firefox has cached.

• https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean

If this has helped to solve the problem then you can remove the renamed cert9OLD.db file. Otherwise you can undo the rename and restore cert9.db.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data