So for the last little while, from time to time, a tiny white box with a shadow effect has been popping up in Firefox, then disappearing after a short time.
I’ve noticed a decrease in speed so I thought it was some sort of spyware, but it persists even after I used AdAware.
Screenshot (arrow added)
Any thoughts as to what it might be?
I’m trying to look at your screenshot, and your server is 404ing on me. 
No idea.
I looked at the site on my computer in Firefox, and even went to the same thread you were viewing, and didn’t see any white ox.
Firefox 3 rc1
It pops up in all different types of webpages, and in a different place on the screen each time. I seem to notice it about once every ten to twenty pages viewed.
Could be related to tooltips
If what Fubaya posted doesn’t help, get Hijack This and post your log here so we can see if you’ve got any browser hijackers going on.
I looked at the source of that page you posted and there’s absolutely nothing weird going on…there’s nothing in the HTML in that space where the box is showing up. But, if what Fubaya’s linked thread is right, it’s not an artifact from the page your on, rather some previous page or open item.
Firefox normally has yellow tooltips, but white “alt” text. It may be an empty “alt” box for a link, but gone astray, perhaps caused by a faulty plug-in.