Inserting an URL in 2.1 Ella does not work (WYSIWYG)

I recently upgraded to the 2.1 FINAL. Everthing went fine, I thought. I wrote a new article, I clicked on the “insert an URL” button in the WYSIWYG-editor and some silly ajax-window came up, instead of the popup-javascript-window that 2.0.7 and all versions before used. All of a sudden the buttons for “insert” and “cancel” are not SHOWN in that ajax-window. I click on the mousewheel and scroll down the window. Happy to find the buttons at the very end of a window that seems not to be dimensioned the right way. But guess what? Clicking on the buttons does not help. So the window stucks, cause when I click on the “x” in the right corner to close the small, shiny, funky ajax-window nothing happens. I have to click the reload button. Nice that the autosave prevented losing the whole article.

But when I try it again and again everytime happens the same. The buttons are not shown, the ajax-window gets stuck and I can hit reload. Guys and girls I am using Firefox in the newest version and I can’t guess what’s going wrong here, but I can guess that things like that shouldn’t happen if a thing is named “final”. Anyone can tell me which file I have to modify by hand for the ajax-window (insert url) will work properly?!