This morning when I logged on to my yahoo mail and entered my inbox folder avast noticed a silent upload attempt to my machine from an iframe on the page. Once I confirmed I want the connection to be stopped the blank part on the far right side turned to the usual 'Firefox can't find the server...' when there is no connection to be made. I've blocked that frame with Firefox extension AdBlock plus which solved the problem.
Now, I'm only reporting this as I have had a friend report the exact same thing as I thought it might be an infection already at my computer causing the problem. On the other hand, it could be nothing as that's the spot that Yahoo often uses to place ads, banners and such so it might be the case of Avast mistakenly identifying that as a thread.