It should work. Try on reinstall Firefox or it maybe a problem within MacOS. https://www.marcozehe.de/2012/04/30/initial-voiceover-support-now-in-firefox-nightly-builds-for-mac-os-x/

You can try to disable multi-process windows in Firefox by setting these prefs to false on the <b>about:config</b> page. *browser.tabs.remote.autostart = false *browser.tabs.remote.autostart.2 = false You can open the <b>about:config</b> page: via the location/address bar. You can accept the warning and click "I'll be careful" to continue. *http://kb.mozillazine.org/about:config *http://www.ghacks.net/2016/07/22/multi-process-firefox/