I have discontinued this extension because I am no longer using Firefox as my main browser and am continuing the extension's functionality on the SeaMonkey version of the extension, called MonkeyFix. I consider that the Firefox interface has strayed too far towards a 'minimalistic' design that I strongly dislike as an advanced user.
I am now recommending that you switch to SeaMonkey's browser and use my "MonkeyFix" extension to get the functionality that was offered by FeatureFix (and more!) That is where I'll be continuing development. Please see my website section on Mozilla addons for more information about switching:
Here is the link to my MonkeyFix extension:
(the old description follows...)
FeatureFix provides numerous abilities to customize the Firefox user interface, as well as enhancements and handy features for developers and power users. These features include: