"agree about the online banking comment, though you're also inclined if you're using a tablet to browse the web you might be also be using it to access ebay or amazon, in which case you're putting out passwords etc."
Using Amazon, eBay, and even your online banking is just as safe on your tablet AllThumbs. This is because they're all secure (https) sites and therefore the data is encrypted in the browser before it's transmitted. Even if a hacker managed to capture that data, he wouldn't be able to make any sense of it.
The same rules apply whether you're using tablet, laptop, or PC - make sure you're on a secure site before handing over your credit card or banking details.
"If I go online via the tablet using the wifi am I not bypassing my firewall security? Is my tablet therefore suddenly waving a little red flag at my IP address and shouting "come and get it!"
If your router has wireless security enabled then you needn't worry about that either.
Also, all routers use something called NAT (Network Address Translation) which means that the router will forward all data to you that you requested and block all incoming data that's unsolicited. It's this mechanism that prevents hackers getting access to your PC or tablet.
If your tablet had a direct connection to the Internet then that's a different story, but it doesn't, so there's no need to worry.
Your tablet is just as secure as any other device that's connecting to your home router. It certainly doesn't need a software firewall.
Hope that puts your mind at ease :)