Open the inspector, reload the page and you'll see:

"Request Blocked

We blocked your request because the IP address you’re using looks suspicious. This issue will usually resolve itself after a short period of time, and you can try your request again. You can also try using a different IP address to see if that resolves the issue.

If you need additional help, you can try one of these support options.

Reference Number: #####"

Thank you for the description. I assume they're using a bot management script which is set to block requests if devtools is open. For such websites, opening devtools in a separate window should work.

Some websites will try to throw you into a loop of debugger statements if they detect devtools being opened, which is harder to work around, but it doesn't seem to be the case here.

Is there an extension or workaround that prevents sites from knowing you opened devtools (without breaking everything else on the site)?

It might be harder to detect if you open it as a separate window instead of docking it to the bottom/side of the window.

Disabling javascript breakpoints usually does the trick. Devtools detection is often done by having a `debugger;` statement somewhere and timing of it triggered

A lot of them dynamically generate new anonymous functions to get around this. Last I looked (~1 year ago I think) neither Chrome nor Firefox supported disabling the keyword completely. Do you know if that's changed?

On Firefox its in the Debugger Tab of the DevTools. On the top right you can deactivate all breakpoints (which includes the debugger statement).

There is a similar button on chrome, but I am not sure if that also applies to the debugger statement.

Yeah, I'm not seeing any evidence to support this...

Do you have a Mailchimp account? It's really easy to test. It happened to me a couple of weeks ago and I can easily reproduce it in different network locations.