Gecko support would be a good as long as it is a completely separate build of Sencha Touch. As others have pointed out solely targeting webkit gives a lot of benefits and keeps the library as lightweight as possible. While Gecko is good, it is still some way behind Webkit and I don't see that changing anytime soon.
As for IE support: don't even bother, if you need it you should consider writing an ExtJS version of your application.
As for those who would say "you should implement your desktop application in a more appropriate framework, like ExtJS, just to get Firefox support": well if you have the resources to write your application twice - good for you =)
The future will show 3 leading mobile platforms: iOS, Android, Windows Phone. Having a toolkit based on Webkit only browsers will prevent Windows Phone from working as their browser will be IE based.
I found a mobile framework "dojotoolkit" which supports non-webkit Browser.
See for example the follow page under firefox, opera or IE:
Best regards at all