Textbox focus issue in Firefox

I’ve noticed a strange behavior of the textbox control in Firefox (55.0.3 – 32bit).
When I click on a textbox the border of the textbox gets selected (colored) but the input cursor and entering text is only possible when I click on the textbox once more.
So to select and enter text two clicks are neccessary !

In Internet Explorer only one click is needed.

When I set the control’s property “focusable” to false it works as expected (with one click)