mouse won't stop scrolling text

I'm using either Firefox or Seamonkey as I like the way I can change font size and text colour in text boxes. I use a particular website (sort of social media) and it has a box showing the Top Ten latest posts scrolling up the screen. What should happen is that if I hover the mouse pointer over a post the scrolling should stop. But with these browsers it doesn't, so I have to chase the text up the screen to try and stop it! Using the mouse pad also fails.

Now, if I use Chrome or IE, this doesn't happen. I've tried different mice but they all fail the same way. Is this a Mozilla problem or is there a way I can sort it? Sometimes, if I open another tab, or close the browser then re-open it the fault might disappear, but that won't always work. I have tried re-installing mouse software.
Thanks.

Update: I discovered that if I 'Restart with Add-ons disabled' then the scrolling text will stop. I've tried enabling each add-on in turn but I cannot tie it down. I think it might be to do with Flash or Shockwave but I can't prove it just yet.