For an SD card and camera Apple offers a Camera Connection Kit. A few of the newest, biggest, fastest SD cards do not work with it, but most do. It will also allow you to plug directly into the camera to download the pictures. Again, it works with most cameras.
There are cheaper third party versions of the kit. As with all cheaper stuff you run a small risk of getting junk. In some cases the price is so low you may not care.
You can not download and use desktop software on the iPad. Firefox does not make an iPad version of their browser; but Google does make a Chrome browser for iOS. Understand that it does not use the same browser engine as the desktop version; but it does offer a somewhat similar experience and the ability to sync your bookmarks with the desktop version. I believer there are a few other tricks it does, but that's all I remember. It's had some good reviews and seems to be popular with other members who prefer Chrome on a desktop.
Almost all the third party browsers, including Chrome, use the built in webkit browser engine on the iPad. That means that if a site does not work well with Safari, it is unlikely to work with any of the others as well.
There are a few exceptions. There are several browsers that use servers to stream websites tot he iPad indirectly. Most of them are intended to let you use Flash, and manage to succeed to various (though not entirely satisfactory) degrees. Even if you are not interested in accessing a Flash site, they often work with other sites that are incompatible with the native browsers. Again, with various degrees of success. I keep the browser Puffin around. Another popular one is iSwifter.
Opera Mini is a server based browser like the above, but does not do Flash. I keep it and Puffin around for the rare site i want that Safari won't work with; with some degree of success.
There are several apps in the App Store that are supposed to work with QuickBooks (in various ways). I would look into those. If you can find a good app that does what you want it will (almost always) work better than using a website.
There is an actual
QuickBooks app. It's for the iPhone, but will work (ugly and with the iPhone UI) on the iPad.
There is also this iPad specific app by Intuit,
GoPayment. No idea if it would work for you, but the reviews don't look shabby.