See:

• https://support.mozilla.org/en-US/kb/error-codes-secure-websites

If you previously created an exception for this website you can 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 -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More 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

I had no previously created exception for this website, this is the first time I visited the site. I get a similar message in Microsoft Edge.

Hi Michael, if your check your router manufacturer's support site, they might mention that this is normal for their device. Assuming it is normal, Firefox typically would let you click the Advanced button and create an exception, or in other words, to trust this certificate even though Firefox cannot verify that it is valid.

I now access my router using its http address (='gateway' address).

Thank you all! The juplink website was no help, I tried calling and got a busy signal once other times the was just silence, and they did not answer my email. I sent the router back.

My comment wasn't clear, sorry. I mean I have to use the router 'IP', e.g. http://192.xxx.x.x/

192.168.0.1 is a common IP for juplink routers you could have tried.