I know it sounds daft. But let me start by saying that — having recently installed a big WordPress upgrade for my blog, having upgraded to the flashy new Firefox 4, and having used Windows 7 for over a year now — it's quite an exciting time using all these new 2010s interfaces.
I've previously resisted tab bars being moved around my browser, the introduction of collapsed menus to save space, and the inevitable "making buttons glow" that seems to come with the desktop application equivalent of the Web 2.0 movement… but lately I'm quite enjoying it.
Anyway, having bitten my tongue and agreed to try collapsing my menu bar into the "Firefox button", what was one step too far was that the tab-bar would now collapse into the window's titlebar when maximised, then drop down to its own line again when the window was restored. This is inconsistent, and also annoyed me because it meant I could no longer safely click on the titlebar to do silly things like close context menus, a habit that I've only just realised I have.
Fortunately, Firefox's immense customisation powers come to the rescue: setting
false solved that in a heartbeat.