has there been a recent google update? within the past few days? 2 of my sites have gone from 150+ per day to about 30 for the past 3 days in a row!!
Google recently updated it's Panda Algorithm about 2 weeks ago, I think that may be the reason of fall.
Google is in the process of rolling out the Panda update world wide. It started last February and has been rolling deeper into the SERPs throughout the year. This could be what is impacting your site.
If your sites are effecting by Panda, then you need to focus on your content. Do proper onpage and offpage of your site, so that Panda should not harm them.
After installing chrome to have a play with it I now seem to have a "google update" plugin in firefox. Is chrome sharing plugins with firefox? Or am I mistaken as to where this plugin came from.