The Flashblock addon for Firefox can be used to block all Flash content on all webpages. This can get very irritating since some websites use tiny Flash widgets for some UI operations. For example, Gmail uses Flash to allow users to attach files to an email. Rather than blocking all Flash content, I would like all Flash content to be available and would like to specifically pick or blacklist the Flash content that I find irritating (like advertisements).
Blacklisting cannot be done with Flashblock. It only has a whitelist option to allow certain sources of Flash content. A blacklisting solution that works is to use the AdBlock Plus addon for Firefox. Hover the mouse over any Flash content on a webpage and a Block tab appears at the top-right corner of the content. Click this Block tab to add this Flash source to the blacklist of AdBlock Plus.
Tried with: Flashblock 22.214.171.124, Adblock Plus 2.0.1 and Firefox 9.0.1