Do you mean you are prompted for credentials when browsing to the Accesspoint using Firefox? If yes, that's all right. Internet Explorer has the possibility to pass your logon credentials on automatically to the server, so although you are being actually prompted the dialog with username and password never show up, because the authentication is done by the browser. That doesn't happen in Firefox, where you don't have a built-in method to pass those credentials to the Server.
Depending on the license you have in the Server, the only authentication method allowed is NTFS, and that makes sense since licenses are assigned to actual users in the Active Directory. Are you using DMS?
So two different things here:
First: Authentication, that means you are able to get to the server and see the list of documents, because your user has permissions to list the documents.
Second: Authorization, that means you are able to open the document once you are in the Server Accesspoint, because you are using a Section Access piece in your script.
There's not as far as I know. Internet Explorer does it because it integrates with the operating system, something that neither Firefox nor Safari do. If you use AD, I don't see any advantage using DMS, since your security directory is managed by Windows.
As an idea that might work, IIS version 7 and higher (Win 7 and Win 2008 and higher) allow users to be identified by means of a certificate instead of user and password (Client Certificate Mapping Authentication here and here). So you can install QlikView to use IIS instead of its own QVWS, set the IIS to authenticate using user certificates and import those certificates in each user's iPad. This way our credentials will be passed on to the server without being prompted.
If you are not familiar with the above, or you don't have support from IT to create the required CA and user certs, I'd rather keep using the dialog rather than starting to make changes to the IIS, that may affect other services in your computers.
You can set your license and authentication to none, should you don't require any authentication/authorization from the users. If that's what you need, check with your QlikTech sales rep in regards to the IAS, because in the EE and SBE Server license editions, licenses need to be assigned to actual users with their corresponding permissions (either AD users or DMS) in the security directory in order to get to the Server, read and open documents.
So in a "normal" environment, you always need to pass credentials on to the Server, but how this happens may be completely transparent from the users client (as it happens with IE properly configured). And the reason for that is that the Server needs to make sure the user accessing has a license and hence, is allowed to see its contents.
I have not tested, but you may create some kind of "proxy portal" that can identify client MACs or IPs and associate them to users. The certificate mapping is probably easier to acomplish and manage, though.