Disable extension install delay - Firefox

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.