Unfortunately the addon I see on AMO looks like it's based on WebExtensions, which TenFourFox doesn't support. If there were an XUL or XPCOM-based version (or were in past versions), that could possibly be coaxed to operate (I'm not familiar with this particular component myself, though).
My js Fu is very limited, and I haven't really kept track of all the developments in the last, um, 20 years -- but it sure looks like they are pulling in all that is new and great in js frameworks... maybe I'll just deinstall the thing, and look up passwords offline. Would have been nice, though.
Yes, it would be a huge deal to backport WebEx support. Unfortunately I'm pretty sure it would be mutually incompatible with XUL/XPCOM extensions, which I definitely do want to continue supporting. If you do find an old-style extension and that doesn't work either, though, let me know -- it could be a deficiency we can fix.