I have a custom web application running on an older version of Firefox. I control dozen or so the machines that run this web application. I have put off upgrading the OS and Firefox version on these machines as the HTML5 context menu functionality has been deprecated. From searching online, I believe there is either an add-on to Firefox or a boolean parameter that can be set that brings this non-standard feature back into Firefox. I would like to run Firefox 111 and newer, and ideally context menus would operate just as they did in older versions to minimize new development. If anyone has experience and/or a suggestion to share it is much appreciated!
dom.menuitem.enabled is/was the setting to enable in the about:config page to keep support for HTML5 context menus. But I’m not sure if the setting still works, or if support has been completely removed now(?)
But it’s a pity it never became widely supported. It was very useful.
Looking in about:config I don’t see that setting listed. If it’s not listed does that mean it’s no longer available to set? I actually have never set a Firefox parameter so not sure if you can just add values into the list adhoc?
I think you can. But it wont help in latest Firefox version it seems. I already had setting enabled in my Firefox 112, and just tested example on https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/contextmenu … It didn’t work 
However it does work with my Waterfox G5.1.4 browser based on Firefox ESR 102 (Not sure if that means it works in all browsers based on Firefox 102 ESR, or if it is just something that Waterfox has decided to keep alive).
I didn’t know about Waterfox. That suggestion is very helpful. I’ll give it a try later on today. Thanks Stig!
Please note that Waterfox (or any other fork) is not supported or recommended by Mozilla.
If you need something that works in ESR (which Woterfox is based on), you could use it directly: https://www.mozilla.org/en-US/firefox/enterprise/