What is this little artifact in Firefox?

[brandoncolorado]

I am using Ubuntu 8.10. In Firefox, i randomly see this little rectangle artifact when I am loading pages.

What is it?

*Screenshot attached*

[pi.boy.travis]

Does it go away after the page has loaded?

[brandoncolorado]

Does it go away after the page has loaded?

It seems to, but I can't reproduce it yet. It just seems to happen randomly. When there, it is usually there for only about 2 seconds.

[brandoncolorado]

I just checked it, and it disappears about 2 seconds after the page is finished loading. It doesn't show up after all page loads.

[4Orbs]

Maybe one of the Firefox plugins doing its job by blocking an ad or script? That happens on mine also occasionally but I never dug into what it might be.