The reason that the double click feature doesn't work on an unselected tab is to prevent you from accidentally closing a tab when you are trying to select it. For example, if you are trying to change the tab and accidentally make a second click, the tab would close if you didn't have the delay.

As for the issue with clicking "too fast", I wasn't able to reproduce the issue. I was able to click as fast as I could and it would close the tab. The exception of course being directly after selecting the tab. For example, you can't triple click an unselected tab (1 to select it and 2 to close it) to close it. There's a small waiting period after switching tabs where the double click feature will not close the tab. This is again to guard against accidental closing.

If you are having consistent issues getting the double click to close a tab if you are clicking fast, try running Firefox in Safe Mode. It's possible that something is slowing Firefox or an extension is conflicting with Firefox, making the program fail to detect a rapid double click.

Hope this helps.

I know this might not be a suitable workaround, but I'll mention it as an option:

If you middle-click a tab using the mouse scroll wheel it will close even if it's not the currently active tab.

I have never used that closeTabByDblclick feature. Instead I use middle-click to close a Tab.

Wesley Branton said

As for the issue with clicking "too fast", I wasn't able to reproduce the issue. I was able to click as fast as I could and it would close the tab.

This is strange, but after a while I also did not notice this problem, before that I climbed in the mouse settings but I do not think that it affected. Strange, but now there is no such problem. :)

Thanks to everyone for the answers, I was disappointed in double-clicking on the tab. Middli click on my mouse to loud and press hard. I find the addon "Double Click On Page Closes Tab" and while it suits me.