Note that Download Squad’s instructions are for Windows; Mac users will need to follow a slightly different procedure:

Go to Tools -> Add-ons

Locate the “Feedback” add-on

Click “Disable”

Restart Firefox

I have to say that I agree with Zukerman; I don’t think that you should disable this add-on. The reason that companies like Mozilla allow us to play with pre-release software/betas is for us to test the product out and for them to get feedback so that they can improve their products prior to launch. A button built into the product seems to be a pretty good mechanism for gathering that feedback. However, if it really is bugging you, now you know how to get rid if it.

Let us know how you’re getting on with the Firefox 4 beta in the comments.