Environment:

• uBlock Origin 1.6.0 (self-built from 64e9eb7 using the included make-firefox.sh script)
• SeaMonkey 2.39

Under "3rd-party filters" it does not work to add a custom subscription pointed to a resource: URI. The subscription I used for testing has many valid filters (including both blocking and cosmetic), but uBlock Origin reports this:
resource://[path/to/subscription].txt : 0 used out of 0
and clicking the link for the uBlock Origin asset viewer on this subscription, gives a blank page.

It seems that what matters here is only that the subscription be at a resource: URI, not a problem with the particular subscription. (FWIW Adblock Plus correctly handles the subscription from its same resource: URI.)

I tried to add the subscription by replacing the entire text of the textbox under "Custom" with the resource: URI; then select "Parse", then disable all other subscriptions (except "My filters") then "Apply changes".

Testing in a clean profile, installed only uBlock Origin and a simple addon that only creates a resource: URI pointer to a subdirectory of the profile; and all settings other than mentioned above are default. (And I restarted the browser after installing uBlock Origin and before testing.)