I had a simlar problem, but on 19.10.
Finally I got things a sort of solution so this note may help some people! I was/am running Ubuntu 19:10 with Firefox 73 on top. NO two ways about it Webex is basically incompatible with FFX 73. The reason is that some of the dlls that Webex uses are 32-bit and multiArch doesn't work properly - yet.
Trying with FFX ESR wasn't much help since openJDK8 gives problems and Oracle doesn't support the equivalent release of Java any more.
Even trying running it through VirtualBox or Wine didn't work. VirtualBox snagged on the JDK8 problem. Wine gave its own issues.
The only viable solution (other than running over Windows) with current FFX over 19:10 is to join the meeting via the Webex web app, which doesn't require downloads. It worked for me with five participants in one meeting and about fifteen in another. The main limitation appears to be bandwidth.
What you appear not to be able to do is host meetings as of now.
Hopefully the issue with the dlls will be sorted sometime soon!