I have the same problem on Firefox with the sites using Flash Player, since I have updated Avast from V 2013.8.0.1504 to 2014.9.0.2011. All was Ok before.
I will try later (I am not at home) the WebShield.ini change from Alikhan/Zdenek

System : Win 7 home french, Firefox V 26, Flash Player V 11.9.900.170

what I am sure, is that I will wait before updating Avast (from V8 to V9) on my other PCs.

NB : What is strange is that the V8 version I had (V 2013.8.0.1504) is not refrenced in the “Avast Release History” (see www.avast.com/fr-fr/release-history?utm_source=prg_fav_80_0&utm_medium=prg_lnk&utm_campaign=free2paid&utm_content=prg_fav_fr-frupdate_info-link&p_var=.%2Ffa%2Ffr-fr%2Fupdate_info-link&p_elm=66&p_lex=198&p_lid=fr-fr&p_lng=fr&p_lqa=0&p_lqe=0&p_lst=0&p_lsu=24&p_pro=0&p_bld=empty&p_vep=8&p_ves=0&p_vbd=1489&p_hid=e161cef0-bed9-49c4-adf9-0a27f0819f10)