Insert link/edit link not highlighting in WordPress blog theme

I have newly created a free WordPress blog account and started writing my first post. What I notice is that the insert link/edit link never gets highlighted, which means none of the external links can be inserted. I tried changing several themes and the result is the same.

I have attached a snap shot of the editing page to give an idea of the issue. Is this something to do with my browser or WordPress issue? Any possible solution?