Bad Request Error when adding to cart in Firefox

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When trying to add a product to the shopping cart with Firefox, I sometimes get the following error:

"Bad Request - Your browser sent a request that this server could not understand.
Apache/2.0.63 (Unix) mod_ssl/2.0.63 OpenSSL/0.9.7a DAV/2 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 Server at sizzle-sticks.com Port 80"

It seems to only happen when browsing with Firefox. I can't duplicate the problem when using Internet Explorer or Opera. I'm now using Firefox 3.6.12, but it was happening with Firefox 3.6.11 also.

It seems like you can USUALLY add the first product without a problem. The problem arises when you try to add the second product. SOMETIMES, shutting down the browser and starting over clears the problem, and allows you to add the first product again (but still not the second.)

One more interesting thing: the second product (and third, fourth, etc) DOES get added to the cart even though it shows the error. When you use the back arrow on the browser, you'll see the item in the cart.

You don't have to be logged in for the problem to show up. The website is sizzle-sticks.com.

This is my version and basic server information:
Zen Cart 1.3.8a
Patch: 1::
Database Patch Level: 1.3.8
v1.3.8 [2008-06-18 18:09:30] (Fresh Installation)
v1.3.8 [2008-06-18 18:09:30] (Fresh Installation)
PHP Version 4.4.9
System Linux rs2.whbdns.com 2.6.30.5-whb #1 SMP Tue Aug 18 01:15:23 GMT 2009 i686

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I posted this almost a year ago, and have had over 200 views, but no replies. Am I the only one who has had this problem? I really need some help!

[Vger]

I really need some help!

Well, let's be honest here, it can't be that urgent - your previous post being at the end of October last year.

It's most likely caused by a corrupt cookie stored in Firefox. Clear all cookies and see if it solves the problem. One thing is for sure - it's not a ZenCart problem.

Vger