Secure Connection Failed If security.tls.version.max at 4

[WildcatRay]

try this - close firefox, rename cert9.db/key4.db in cert9.old/key4.old (delete cert8.db/key3.db if present), restart fox. maybe cert store damaged.

Thanks, but you missed an edit of mine in my OP.

After refreshing Firefox without fixing things, I uninstalled and then reinstalled Firefox. When that did not fix things, I copied a profile from 1 of the unaffected computers to that 1 with the issue and it, too, did not fix things. That should have replaced a corrupted cert store with a good one and yet things apparently were not corrected.

[morat]

Could it be a problem with a "man in the middle" (e.g., security software that filters your connection, proxy server, or malware)?

I'm having a similar problem with Chrome 70 and Avast Antivirus. Avast recommends uninstalling and reinstalling the latest version.

More info
http://www.chromium.org/Home/tls13

[jscher2000]

Could it be a problem with a "man in the middle" (e.g., security software that filters your connection, proxy server, or malware)?

Then, why not all 4, not just 2 computers?

I'm suggesting something to investigate considering the evidence provided. Maybe only two of your computers are infected. I mean, affected.

[WildcatRay]

Perhaps the answer was some "man in the middle" issue. I had my AV prompt for an app restart to install an update. Afterward, I set security.tls.version.max back to 4 and, so far at least, the page now loads without error.

Ironically, neither Chrome, nor Vivaldi had any problems.

[Brummelchen]

as i wrote - firefox has its own cert store, edge, ie and chrome, vivaldi and other share system store. that info is vital for identifying issues to this.