Fedora 42 x86_64, firefox 145.0 Using FontForge, I created a custom Unicode (NO 8-bit pi encoded) version of the Symbol typeface with a new font-family name. One of the customizations, the serif versions of the "Registered Sign", "Copyright Sign", and "Trademark Sign" are available by the official Unicode codepoint. The sans-serif versions are available by the official Unicode codepoint *directly* followed by U+200B (Zero Width Space - ZWSP). This is done with the OpenType contextual alternative feature (calt). It works in the FontForge metrics test window and it works in Chromium and derivatives but it does not work in FireFox - either the Fedora 42 build or the generic x86_64 build from Mozilla. I tried the CSS setting: body { font-feature-settings: "calt" on; } No dice. I believe that's the right CSS setting because if I change the "on" to "off" then it no longer works in chromium and derivatives, so that does seem to the be the CSS setting that controls whether or not the feature is enabled. All my searches on the web seem to indicate the FireFox supports calt however all the examples I see are related to swashes which are enabled/disabled by a different CSS setting. I did see some indication that some apps only allow calt for certain language scripts, typically Arabic and/or Greek and/or Latin and perhaps these legal glyphs aren't considered one of those scripts? Does anyone know why the calt works elsewhere and not in FireFox? Yes, I tried without any extensions or plugins enabled.