Firefox displays a number of different warnings, so I'm not sure which one you are seeing. Perhaps it's the "mixed content" warning about some elements of the page not being retrieved over a secure connection? Or the insecure form submission when you are sending information that isn't encrypted??

Next time, could you make a note of the specific error or grab a screen shot? (How do I create a screenshot of my problem?) Note: if you plan to post the screen shot, blur out the details of your transaction.

You do not get that warning about mixed content any more with Firefox 19. It has been removed from the browser.

The warning I get is the insecure form submission when you are sending information that isn't encrypted warning.

Edit: In the below, I assume you meant that you got the error when submitting from a secure page to an insecure page. Unlike some other errors, I don't think you can disable that one.

Well, the error means what it says. Many sites have you shop on a standard, non-secured connection between it offers slightly faster performance. They encryp (using HTTPS) only the pages where you are logging in and transmitting your purchase card information or other sensitive information. After your purchase is processed, they forward the less sensitive information, such as your itinerary perhaps, to an insecure (HTTP) page.

Obviously you should never get the error when submitting a form containing sensitive information! In that case, cancel the submission and use a different site.

If you don't like the way Expedia switches between secure and insecure, you might be able to choose a setting on the site to keep everything secure. Or you might choose a different site that keeps everything secure. You might want to submit this issue to their feedback or tech support.