Hi Clem, I don't have any way to test this, but you could try:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste network.security.ports.banned.override and pause while the list is filtered.

This preference does not exist by default, so you would expect to get a bar with

Boolean Number String +

(3) Assuming the preference does not exist, click the radio button next to String and then click + to add the preference and open an editing field. Type or paste your port number and press Enter or click the blue check mark button to save the change.

(4) If the preference already exists and has a value in it, double-click the value, add a comma, then type or paste your port number and press Enter or click the blue check mark button to save the change.

Firefox monitors two prefs in about:config to add a new port to the block list and to override an existing block, both use a comma separated list.

• network.security.ports.banned
• network.security.ports.banned.override

• https://searchfox.org/mozilla-release/source/netwerk/base/nsIOService.cpp#1543
• https://searchfox.org/mozilla-release/source/netwerk/base/nsIOService.cpp#1627

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox