Here is a quick guide on how to remove Firefox's error message when trying to access resources on restricted ports, such as port 87:
This address is restricted
This address uses a network port which is normally used for purposes other than Web browsing. Firefox has canceled the request for your protection.
Open Firefox's preferences by typing the following URL in Firefox:
You might get a warning that you are entering sensitive configuration, if so, continue with caution:
In the filter box, type in:
I have noticed that simply toggling this to "false" or deleting it does not actually permit restricted addresses.
Instead what we need is to have this preference as a String of comma separated list of ports.
So if you already have this preference (as a Boolean), simply delete it:
Then recreate the preference as a "String":
Next add "87" as the value and save it:
Afterwards it should look something like this:
If needed, you can also specify multiple ports either as a range such as "1-1024" or even combine with a comma separated list such as "87, 150-300, 350, 400, 450-500".
Just refresh the page and you should have no problems accessing the resource.