A series of issues.

Date and time were the first things I checked. I disabled the firewall and reset it to defaults. No other security software is installed. No proxy. Browser has no proxy. I deleted cert9. I didn't touch any settings. 13 There was a browser update, but on the old PC with Windows 7, a different browser version that hasn't updated yet also can't access the forum. Edge can't get in either. As for digging into certificates, I don't understand at all what's there or what it's for. That's all I've got.

Does the site open from a smartphone?

If possible, please record a video of your actions when the error "Error establishing a secure connection" appears, along with screenshots from the "Network" tab, which you can access via CTRL+SHIFT+E in Firefox (it needs to be opened before the site loads, otherwise it will be empty, or open it and reload the page — this will be noted in the tab itself)

In the screenshot you sent, the browser actually shows that the secure connection has been established, so the error contradicts itself.

If the connection were insecure, there would be a completely different icon — a "crossed-out lock" — which appears when you open a site with a problematic certificate, or without any certificate over HTTP, which happens, for example, if you open the website of the Central Bank of Russia http://cbr.ru