Using Firefox for Mac, I get a tiny dot in the wikipedia search box that moves as you type text in it.
Any clue why that’s there?!
No idea, but I get it on Firefox for Windows. It looks like a “`” to me. It’d been bothering me as well.
It’s part of the oddly-shaped insertion point. It blinks with the vertical line it’s attached to. Dunno why it’s not a simple vertical line or an I-bar like Word uses, but I think that’s all there is to it.
I’m using Firefox for WinXP and no funny blinky insertion point for me. I get a plain blinky vertical line. Are you sure ya’ll don’t have a fly speck on your monitor. 
It’s the BiDi (bidirectional) caret. It appears in Firefox whenever the text input you’re working in is formatted to accept right-to-left languages, like Arabic or Hebrew.
Jimminy Cricket, what people on this board know! (I too, use Firefox, and have seen that symbol in other text boxes, and was curious.)