It is possible that there is a plugin on that page that gets focus and in that case scrolling might not work anymore.

Thanks for the response Cor-el but it doesn't seem to be a focus issue. Have also tried in "Safe Mode" which disables plug-ins but that didn't fix things.

The only pattern I've found is that I can always scroll when my mouse is in the page margins but once I enter the body of the site (even if it is blank) scrolling will frequently stall. The easiest way to get it to resume is to move towards the edge of the page and scrolling will reinitiate itself but this is not always necessary. It doesn't seem to be influenced by cursor type (arrow, hand or text cursor or indeed tapping) full screened or windowed.

I will continue to investigate but it makes using FF4.0 just about unbearable.

Safe mode only disables extensions (Tools > Add-ons > Extensions) and not plugins like Flash or the Windows Media player that plays media files. Plugins would have to be disabled via Tools > Add-ons > Plugins

Thanks cor-el. Have now disabled all the plugins (and extensions) and restarted but unfortunately the problem still remains.