Some people don’t like change. If you’re happy with Firefox 3.6, then upgrading to Firefox 4 could be a culture shock. What happened to the menu bar? And what is that ghastly orange Firefox button in the top left-hand corner for?
It’s possible to tweak Firefox 4 to make it look almost identical to the previous version, but there are some things you can’t remove without the help of third-party software. The orange Firefox button is one of these: this Add-on quite simply removes it for you, and you won’t even need to perform a restart after it’s installed.
And that’s basically it, except to say that if you like the idea of the Firefox button, but don’t like the colour, you might prefer to try one of two related Add-ons by the same author instead: App Button Color, or App Button Clear. As you can probably guess, they too do exactly what they say on the tin.