Redirect problem in one browser, VCACHE problem in another

I got a note from a customer (we're using 1.5.4.1) that when he tried to login to our site, Firefox told him the site was trying to redirect in a way that would never complete. I was able to replicate the error in Firefox, cleared the cookies, and it was fine. However, he reported he also got the error in Safari, IE and Chrome on Windows 8, and that Safari gave him this:

"Notice: Undefined index: HTTP_REFERER in /home/343/public_html/343/vqmod/vqcache/vq2-catalog_controller_account_login.php on line 66Warning: Cannot modify header information - headers already sent by (output started at /home/343/public_html/343/index.php:104) in /home/343/public_html/343/vqmod/vqcache/vq2-system_engine_controller.php on line 28Warning: Cannot modify header information - headers already sent by (output started at /home/343/public_html/343/index.php:104) in /home/343/public_html/343/vqmod/vqcache/vq2-system_engine_controller.php on line 29"

I had someone report the redirection error once months ago and using another browser worked until we figured out to clear Firefox's cookies for the site. I'm looking for a way to stop that completely if I can and figure out why this new customer is getting the additional Safari error.

Thanks!