https://www.ghacks.net/2016/05/23/install-google-chrome-exte... might be worth a look. Haven't tried it, but the claim is that it mechanically transforms Chrome extension metadata and containers into Firefox's formats, and supports automatic signing via AMO for permanent installation. Assuming that, the rest should Just Work modulo possibly still incomplete WebEx API support in Firefox. If you try it, I wouldn't mind hearing how you get on.

Holy hell, it just worked! That's amazing, thank you for that. I bet it helps that the extension is just redirecting and collecting stats, but everything seems to work fine!

That's great to hear! Most welcome, and glad to know there's a plausible option - with the rapid deprecation of the old API, this might save a lot of time. (I can't rely on replacing all my old extensions with shell oneliners, after all...)