Hi,
I would be so grateful if anyone could help with this.

I use Linux Mint 17 MATE and Firefox, all full and upgraded frequently. Often - not always but usually, I find the complete window suddenly scrolls to the bottom, showing the end of the window and making the programme unusable. Once it is scrolled down to bottom, it can only be moved up by using arrow keys and not by mouse. The problem may not do this straight away, maybe after two minutes, may be ten. It affects Firefox, my usual browser, Libre Office word processor, and some folders - but not all. Sometimes it can be halted by re-starting. Sometimes it goes away by itself. It makes no difference whether I am online or not, nor if I disable my connection.

When I began to type this in your window, after a few letters the typing was entered at the bottom of the window - even though I had not yet dropped the scroll bar. I have had to type with text editor - which has once or twice been affected by the problem itself - and copy into your window. What causes the scroll-bar to drop down?

I would suspect a stuck key on your keyboard. Remove the keys and clean the keyboard.