Can't enter username in form with Silverlight I have had to install MS Silverlight in order to carry out online marking of examination answers on platform 'RM Assessor'

The RM Assessor login page appears, with a username window and a password window. I know both of these.

The problem is this: when I type in my username, absolutely nothing happens. However I can type in the password.

There is a security setting in OS X which causes the problem and which can be reset. How do I know? I am marking exams for a second organisation which also uses RM Assessor and Silverlight. It took 90 minutes on the phone but eventually they found someone who knew what to do. But I can't contact them again. 

So I'm stuck again.

MacBook Pro 15" Retina September 2012, OS X Yosemite
Lets assume you use Firefox browser (and have the plug in installed).

If the cookie information is displayed at the bottom of the login page, click on the Close button to hide the cookie information and then click back into the Username box. You should now be able to enter/type your usename.

If the cookie information is not displayed at the bottom of the login page, click on Tools and then Add-ons. This will open a new tab where you can view the Plugins you have installed. Once the new tab has appeared you can close it and then click back into the Username box. You should now be able to enter/type your usename.