Strange issue with Firefox and Nightly

[X11]

This one is a little hard to describe

Basically firefox and nightly on debian stable are having a strange bug. When I press shift-u a little box appears in the middle of the browser window with "default/ios/android/custom" written in it, cut off at the end. I can't get a screenshot of it, it disappears when any key is pressed including print-screen. This bug is incredibly annoying, I can't use an uppercase u in any text input in the browser. I tried to get a video of it but I'm not sure where Xvidcap saved the file. It's a rectangle in the middle of the screen for text input, with text already in there. Further keypress will blank it and start new text in that box. I've tried downgrading from Nightly to Firefox but the bug still exists.

[X11]

Update: The box that appears on the screen seemed to reload the page after pressing enter. I just had a look at phpinfo(); on a script. The text you enter becomes the browsers user-agent string on the request to reload to page. Incredibly strange stuff.