Orion had re-implemented support for the browser extensions APIs in WebKit. Though WebKit more recently opened up its built-in addons support to third-party browsers that use it.
Do you mean Firefox on iOS? If so, that would require the firefox-ios project to adopt BrowserEngineKit, which is relatively new. Firefox for iOS WebExtension support is being tracked in https://bugzilla.mozilla.org/show_bug.cgi?id=1497374