Usually I can spot a site by going through the tabs and looking at the status message bar. Although there is no change in the appearance of the tab, the status message is changing every couple of seconds. I think it's just poorly written or overly aggressive scripts, usually to an ad site, or google. The addon, No Script, or Web Developer to stop javascript completely, might be the only solution. I'm thinking of having two profiles for Firefox. One, with all the fun stuff, and one for work. The one for work, won't enable javascript.