Was "Interface Builder" used when writing Firefox and Skype for Mac

Or did used API calls at their source codes.

(Firefox is a open-source application, but Skype is not. When I looked at Firefox's source code I couldn't see any ni开发者_如何学编程b (xib) files.)

Skype 5.2.0.1523 has 87 nib files in its en.lproj folder in the package. To see them, right click on the app bundle and select "Show Package Contents". This makes it quite plausible that Skype was indeed written with Interface Builder.

On the other hand, as danielkza points out, Firefox uses a custom layout system (XUL) based on XML and Javascript for all platforms.

Certainly not for Firefox, it uses a custom layout system (XUL) based on XML and Javascript for all platforms.

It's possible it was used for Skype, but you'd obviously need to ask one of their developers about it to be sure.