I don't think we can tell. For motion it's usually the Adobe flash nightmare. And Adobe has the plugin for installing so long as you click on "other operating system" or "other browser" (than IE)
Yet, I might install the legitimate adobe flash-trash from the legitimate Adobe site and still get the error on some sites. Other times it works.
I'd never click on 'download Adobe flash' button on any website that tells me to. Just don't trust them.
If it doesn't work, the site is bad or how the site implements the use of flash code is bad. I usually give up, try another site that works. Lost sales to badly designed sites
A browser I use, Opera, and Avira's antivirus will warn if I click on a site or a link I shouldn't, still it is a bit of concern here considering how fast the scumware writers defeat all security.
Perhaps someone here will tell us exactly how to deal with motion and flash and what's safe, as what I just wrote are my obesrvations and a bit of frustration.