However, I have a problem accepting that it's the scenario you describe.
Okay, let me put this in simpler terms. 🙂 When you fill out a webform, you're just inputting text on an HTML page (okay, some webforms can be generated via JavaScript, but the underlying issue is the same). What gets passed back to the server when submitting the form is just text, usually through a POST statement that contains the form field identifiers and values contained in those form fields.
There's a 99% chance that the form has a validation script to verify that the data you entered is in a format that is acceptable to pass back to to the server for processing. That validation routine runs through the web browser on your computer. Validation can take place either when the form is submitted, or interactively as you fill out the form through the use of AJAX or JavaScript.
It is extremely rare that websites will use server-side validation as it generates an excessive amount of traffic and puts the load on the server to execute the validation.
As I said, "The reason a different browser will work where Safari may not lies in the rendering engine the browser uses. Safari uses WebKit, Firefox uses Gecko; _either will render/execute code differently._" The fact that your postal code entry gets rejected in Safari but not Firefox tells me two things:
1) The validation script doesn't execute properly in Safari, and;
2) The web designers for the site(s) have not run _comprehensive and thorough_ browser-compatibility checks.
Certainly, you should notify Apple of the problem as JasonFear suggested, since this is most definitely a Safari bug. But you should also notify the site owner of the issue as well.
Good websites should be pushing for standards-compliant design and function. While Safari may hold a small share of the browser market, it is large enough not to be ignored when running compliancy checks on websites. When I'm working on website designs, I check the pages in as many browsers as possible, both Mac and Windows, to make sure that everything functions as consistently as possible. They should be doing the same.
-Douggo