'Content Encoding Error" in Firefox

Any idea why I am getting this error and unable to access ARRSE using Firefox 11.0 on Window 7 x64?

"Content Encoding Error
The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression."

This is only happening in Firefox, all other browsers seem to be OK. Problem started this morning. Have tried briefly disabling firewall, clearing cache, restarting laptop and modem, makes no difference.

PS also tried Firefox in Safe mode - add-ins disabled - still no joy.
Thanks as ever GCO. The problem turned out to involve something called Speedbit Video Accelerator which comes as a bundled option with RealPlayer (yes I know). When in Firefox, in my setup, no access to sites (not just ARRSE) which have file compression such as gzip. Unwanted program removed, problem solved.