On some systems you can just press F12, but the alternate route is to click on the three vertical dots in the upper right, then select "More Tools", and then "Developer Tools".
Since the release of Mumble 1.2, users identify themselves with a certificate instead of a password.
If the user chooses to do that, and enters a valid e-mail address, it is possible to upgrade to a Class 1 or Class 2 certificate for the same e-mail address without the murmur server forgetting the user.
In other words it is strongly recommended to use a valid and usable e-mail address even if you start out with a self generated certificate.
See the following pages: New and Noteworthy, Known Issues.