Disable extension install delay - Firefox
Jesse Ruderman gives us reasons for not doing this EVEN IF YOU ARE AN ADVANCED USER. Please read it before proceeding.
Change the necessary lines in "toolkit.jar/content/mozapps/xpinstall/xpinstallConfirm.js" to:
_installCountdown: 0
And
this._installCountdownInterval = setInterval("XPInstallConfirm.okButtonCountdown()",0);
You can access "xpinstallConfirm.js" by unzipping "toolkit.jar" in your "/path/to/Firefox/chrome" folder to somewhere else, keeping directory/folder structure intact. Modify the JS file and repackage everything with the folder structure it originally was stored into ZIP file. Rename the ZIP file to "toolkit.jar" and rename the original "toolkit.jar" to "toolkit.jar.bak" (as a backup). Then place the new "toolkit.jar" into "/path/to/Firefox/chrome".
Stolen from Cusser.
Note that the extension MR Tech Disable XPI Install Delay does this. Also, the preference "security.dialog_enable_delay" should technically do this also when set to "0" but it currently is bugged.