In my tests, clicking on the port triggers the button, which then opens a chrome: URL that doesn't exist in the older versions of Firefox for which we display the port. And since users will want to click the port number to select it (in order to copy/paste it), they are likely to experience this problem if they use the remote port at all.

That's peculiar, since hiding the button sets it to `display: none`, which should completely remove it from the DOM. But in any case disabling the button after hiding it seems to work around the problem:

```
               //       and show allocated debugger port on previous firefox releases
               if (message.hasConnectDevtools) {
                 $("#show-debugger-port").hide();
+                $("#open-connect-devtools").prop("disabled", false);
                 $("#open-connect-devtools").show();
               } else {
                 $("#show-debugger-port").show();
                 $("#open-connect-devtools").hide();
+                $("#open-connect-devtools").prop("disabled", true);
               }
             }
             else {
```