Cannot enter text in Firefox

I have two Firefox windows open, each with about six tabs. At some point during the day it will become impossible to enter text anywhere in one of the two windows, in the URL bar, search, or input ares in the tabs, eg Gmail or Google.
I was having the same problem in 13.06. It only affects one window, so far.
The mouse still works, for instance if I try to compose an email I can’t enter any text, but I can close the email with the mouse.
I can do a right click “paste & go” in the URL bar, or “paste & search” in the search area.

Is there anything I can do about this or is it a bug? I have just run all updates but it makes no difference.

This sounds indeed like a strange bug. Do you have any special keyboard like bluetooth or other wireless ?
If its an usb keyboard did you try to plug it in another usb port ?

The keyboard monitor and mouse run through a switch so I can use them on 2 PCs. It’s been like that for a year, long before this problem arose, which was after I finally got round to installing 13.06, in Nov '13 I think. It takes a few hours, or maybe a certain amount of use, before the effect kicks in.
I’l try and disentangle the USB cable from the birds nest tomorrow and see what happens. It’s nearly midnight here now.
Well. The effect has now gone away. The last thing I did was the “paste & search”, after which I still could not insert via keyboard, or get focus with the mouse, although the mouse was still effective in other ways. I’ll see what happens tomorrow.

Tomorrow. I’ve connected the keyboard, monitor and mouse direct. The switch seems to combine keyboard and mouse into a single USB cable.

Bypassing the switch did not fix it. I’ll just have to log out and in. I will try uninstalling unclutter, which hides the mouse pointer when it stops moving for a while.

I uninstalled unclutter and the problem has not repeated. Inference being that unclutter was the cause.