Mysterious little rectangle appears

As of a week ago, there is a mysterious little rectangle that sporadically appears beneath my pointer arrow and follows it around the screen. I checked my mouse trailer settings and that feature is turned off. If I move the arrow quickly away from the little rectangle, I can outrun it, and it will disappear for a few seconds, then reappear again beneath the arrow. The little rectangle has a very tiny drop shadow. I'm attaching a screenshot in case you want to look.

It bothers me when something occurs onscreen that I didn't put there. Nothing bad that I know of has happened with my computer, other than it runs a little slow at times, but I fixing to correct that.
(I'm getting ready to swallow Major Attitude's cleanup/cleanout pill).

I wanted to throw a hail Mary in case someone here might instantly recognize what this is.

Any constructive comments would be welcome and appreciated.

Thanks,
4deucer

 

Read recently that firefox isnt the most secure, which was a shame I loved it. something about some keys that werent even considered or protected. I no vey little of the finer details and this may have been addressed in the most recent release.

Downloaded any free apps recently to this p.c...

A little suspicious though, dont have a key logger or similar app installed?

L8r

 

Thanks necro61 and plodr.
I only use FF, and right now I'm using version 3.5.7.
No key logger app that I know of has been installed.
I have recently allowed some "permissions" to several apps on Facebook.

Anomaly does not occur off the internet, so I'm assuming it's browser-related.

Just so you guys know I'm not sitting on my can waiting on someone else to figger out my troubles, I googled "mysterious rectangle appears beneath my pointer arrow," and got several returns, some of which mentioned that this behavior has been reportedly related to the Mac Leopard 10 browser.

This finding triggered my memory (and I see that I failed to mention in my original post) that I recently began using Leopard 10 desktop background along with the Rocketdock program on my Windows PC to give the appearance of the iMac, as a test. Not the browser, just a desktop background. So far, other than what I have questioned, there have been no bad effects.

I appreciate both of you and any others who may have expended their time in answering or otherwise.

Here goes with the Major Attitude exercise I mentioned earlier.

Thanks very much.
4deucer

 

That was it, plodr, and thanks for the suggestion! Now, it's to the task of one-by-one's, to see which add-on is causing the pesky little rectangle! :major