Right now, if you want to do this you have to go for things like user stylesheets or Greasemonkey scripts, or other specific Firefox extensions, all approaches that are sub-optimal at best because installing them takes a certain browser and isn’t really “one-click” easy, and there’s also the risk you disobey the Google terms of service.
An officially supported Google Apps add-on API – sort of like macros for Microsoft Office, but something different than Google’s current Apps API – on the other hand would guarantee easy installation. Additionally, Google could then link to these extensions right from the specific product pages. E.g. a link at the top right side of your Gmail account may bring you straight to the Gmail add-ons of the Google Apps API directory. You could then subscribe to several extensions, Personalized Homepage directory-style, and extend your Google Apps with new functionality.