You already have your answer in that double-clicking on an empty space is not available with tabs on top. Short answer is don't use tabs on top as you've already seen.

You've probably already double clicked and seen that the window maximizes because that is what happens when you double click on the title bar (at least in Windows 7 don't remember before win7), you can tell from the color that you have a title bar there. The developers see that that is an important windows function but seem to not see the band at the top above the tabs as big enough to click on -- so to me all logic fails on anything to do with tabs on top or no menu bar. If fact all toolbar layout after Firefox 2 defies the logic of being customizable because all customization has to be reworked over and over again to undo what is changed and continue to make your own styling changes as before.

I use far less space with all toolbars including menu bar than you see in Firefox4 and up with all toolbars except menu bar.

Look, Fact remains. in previous FF7 beta you could double click on the tab bar, whether it was on top or not, on windows 7, and open up a new tab.

I understand what You're saying about logic and all. but I just don't like how the browser look with the tabs on top. and there is no reason to annoy people by removing this option. Not using tabs on top may be ok for you. but for me it's not.

@Gargar I agree, I don't like the tabs on bottom either. This Firefox extension fixed the issue, plus gave a few extra options. https://addons.mozilla.org/en-US/firefox/addon/tab-mix-plus/

also, if you do not want to add the extension, you can middle click to open a new tab with the tabs on top.

basically the only real solution i found (installing an extension is absurd to solve such a simple issue) was mentioned here: http://superuser.com/questions/261092/how-to-make-double-clicking-the-tab-bar-create-new-tabs

you need to create a chrome folder under your firefox profile folder and add a file named userChrome.css.

in it paste this code:

#navigator-toolbox > #TabsToolbar {
  -moz-binding: url("chrome://global/content/bindings/toolbar.xml#toolbar")
!important;
}

restart firefox and your done. double-click now works as god intended it!

hehe... I wiped my PC a couple of days ago and searched for how to fix the 'double click for new tab' issue and came across the answer that I posted 9 months ago. Oh what a small internet we live in.