1. Why don't you use Ctrl, Shift, or Alt as the allow key?

Firefox already makes use of all three modifier keys (Ctrl opens a link in a new tab, Shift opens a link in a new window, Alt downloads a link).

2. How do I change the popup allow key?

Go to Tools->Add-ons->Extensions->Popup Allow->Options, there you can add a custom allow key.

0.2.6

• Added support for Firefox 3 final.

0.2.5

• Added support for Firefox 2 final.

0.2.4

• Added compatibility with the latest nightlies (2006-03-17 and above).

0.2.3

• Fixed problems with the latest post-1.0 Firefox builds.
• Added an Italian (it-IT) locale, contributed by andr3a of eXtenZilla.it.

0.2.2

• Minor improvements to the custom key configuration dialog.
• Added a Korean (ko-KR) locale, contributed by Kim Hunmin.
• Added a German (de-DE) locale, contributed by Patrick Ruh.

0.2.1

• You can now set the custom allow key in the add-on's options dialog.
• The popup opener menu is no longer shown in newer builds (as these builds come with their own popup opener).
• Removed the automatic install.rdf update link due to bandwidth constraints, from now on you'll have to manually check for updates.

0.2

• Added the ability to open blocked popups by clicking on the icon in the statusbar. Left click opens the blocked popup in a window (only on recent nightlies) and middle click opens the popup in a new tab. The popup whitelist controls are now available on the icon's context menu.
• The Insert key now disables popup blocking as well as the Caps Lock key.
• You can now create a custom allow key by setting the pref popupallow.allow_key to the numeric code of the key you want to use (all of the key codes can be found in nsIDOMKeyEvent.idl in the Mozilla source code).

0.1

• Initial Release.