In Firefox when you are typing in a domain name you can enter just the domain name and then hold down the Command key (on a Mac, probably the Control in Windows) when you hit Enter. Firefox will automatically put a www. in front of the domain name and add a .com to the end.

While this is great, if you live in a foreign country or visit a lot of non-.com sites, this doesn't really help. Luckily Firefox lets you customize the text that it prepends and appends to the domain name.

Type about:config in your address bar and bypass any warnings. The Preference Names you are looking for are:

browser.fixup.alternate.enabled
browser.fixup.alternate.prefix
browser.fixup.alternate.suffix

browser.fixup.alternate.enabled needs to be true for this to work. If it's set to false, double click on it to change the value.

browser.fixup.alternate.prefix sets the text at the beginning of the domain. By default this is www.. If you want to change the value, click on it and enter a new value (you can leave it empty for example).

browser.fixup.alternate.suffix sets the text at the end of the domain. By default this is .com. If you want to change it to match the domain you spend most of your time in, click on it and enter the new domain. Don't forget the period.