There are lots of reliable online sellers of laptops, and online buying is fine, provided you have somehow handled the machine and are familiar enough with it to be sure it’s the one you (or, in this case, your lucky wife) will be happy owning. In the case of Apple, a company whose products are rarely deeply discounted by third-party sellers, it often makes sense for online buyers to use the company’s own online store, at store.apple.com. I have found Apple’s online store to be easy to use, secure and reliable. And you get some minor benefits at the physical Apple stores if you buy directly from Apple.

However, other major online sellers with equally good reputations also carry Macs, and some do shave the prices. One good example I have used happily is amazon.com, which carries the latest MacBook Pros at discounts ranging from $5 to over $100, depending on model.

How can I set parental controls in Firefox?

The Firefox Web browser doesn’t have built-in parental controls. Its maker, Mozilla, notes that version 3.0 or later of the browser does support some of the parental-control features included in Windows Vista. But if you don’t have Vista, or want different controls, there are some add-ons for Firefox that provide these.

Among the ones Mozilla suggests using are Glubble for Families, and ProCon Latte. More information is at: support.mozilla.com/en-US/kb/Parental+controls.

I bought an iPhone and was frustrated that my fingers appear to be too large to type properly on its virtual keyboard. Is there a stylus that works with the iPhone?

I hadn’t realized this until recently, when a friend who bought one showed it to me, but, yes, there is. The iPhone wasn’t designed to operate with a stylus, and the small, thin, hard kind that one uses with, say, a Treo, doesn’t work right with the iPhone. But there are some companies that sell a wider type of stylus with a soft tip that approximates a small fingertip, and is designed for the iPhone.

If you type “iPhone stylus” into a search engine, or an online store like Amazon.com, you will see a number of inexpensive choices. One downside: the iPhone doesn’t have a built-in slot for a stylus. My friend carries his, which has a clip on the end, like a pen, in his pocket.

I just purchased an external hard drive for my PC that connects through a USB port to back up photos and important files in the event of a computer crash. Would this hard drive be affected if my main hard drive crashed? Should I disconnect the external drive when it’s not in use?

The two hard disks are separate devices, and if one has a mechanical failure, it doesn’t affect the other. There might be some scenarios in which a problem with the computer itself, as opposed to a breakdown of its internal hard disk, could affect the external drive. And malicious software could corrupt or erase files on the external drive. An electrical surge could also affect both drives, or fry the computer itself.

Many people can’t disconnect their external drives, because they use automated backup programs, or frequently save files to the drives. If you are doing only occasional manual backups, you could disconnect the extra drive when not in use, so you’d feel more comfortable. But, for maximum peace of mind in case of an electrical surge, you should disconnect it not only from the PC, but from the electrical outlet as well, even if you are using a surge protector.

You can find Mossberg’s Mailbox and my other columns online free at the All Things Digital Web site, http://walt.allthingsd.com.

Write to Walter S. Mossberg at mossberg@wsj.com