This happens only once in a while. I'm also not sure how to replicate this behavior, but it happens often enough to be annoying.
When I want to write a new email a depress "compose" button > a new dialog is up > I enter the destination address. Than when I attempt to click in the message text input box the cursor simply does not appear. Only if I click significantly lower does the cursor appear and let me type. This excessive header space does not however effect the resultant sent message.
I usually use the  personal.zoho.com, on a 3.6 Mozilla Firefox.

I will post a screen-capture when the problem reoccurs.