I found something similar a week or two ago, but couldn't consistently reproduce it, and my colleagues were unable to.
But now that I have your report, I spun up a brand new test site and I reproduced the problem 2 out of 3 attempts (one time it worked okay).
It happens if the error shown in the screenshot appears on the page. It doesn't appear to have anything to do with Blocks, but only happens when Blocks is active.
So I've escalated the problem to the developers to investigate.
Just to let you know, the devs found that there are more generic reports of the same not related to Toolset, but affecting Firefox specifically: https://core.trac.wordpress.org/ticket/52086
Got it- I checked in Chrome and the add user function was working correctly. Strange that it was specifically Blocks affecting Firefox (if I turned off Blocks, I could add users in FF). Thanks for the help, we can close this.