Thank you all for your replies.
I use mu USB3 port to run my dual band Wi-Fi dongle so that port is permanently occupied. The USB2 ports I use for my printer, satnav, camera and everything else I have to connect via USB.
In Device Manager I am not showing any conflicts but whenever I connect a device to the USB2 ports they remain invisible whilst Firefox v30 is installed - satnav, printer, camera, usb extender and so on. As far as the computer is concerned they just do not exist.
I find it very strange myself as to how hardware is being affected and what/how resources are being conflicted but all I can say is that they are. Using the link from the Moderator, I have re-installed v29.01 and everything is fine again. I have Firefox loaded and my USB2 ports are working. I have disabled automatic updates for Firefox though and will probably wait until v32 is released before I try updating again.
I read somewhere that someone lost their touchpad mouse after installing Firefox v30 so at least someone else is in the same boat as me albeit with a different hardware problem.
Most folk seem OK with v30 so it must be down to individual computers/laptops and how they handle resources and software installed. I don't like to be beaten by a machine but sometimes you can only reason so far and the work around for me has been to go back to v29.01.
Thanks again.
Steven