This a old story with a new twist.
Meaning Firefox.
IE has been a issue with the last few versions. Even with some patches to try to fix it.
It is pretty much a random issue with keeping login status.
Some have fixed it using compatibility mode in IE.
The usual response has been use Firefox instead of IE.
I know SD is working on more fixes for IE in the next release.
But if the user is having issues with Firefox I am at a loss.
The only thing that comes to mind is if you updated your site to 7.5.7 then the user should try to delete the cookies and try again.
The user can do that using mystuff-cookies.
The reason I say that is in 7.5.7 cookies were worked on in that version and your user has tried Firefox.
Firefox has always been flawless as far as I know.
Except for a few that have set exceptions for additional security settings.
But anyway with the current version it is possible to have a (corrupted?) cookie that needs to be reset. Since the new version has a new setting for cookies that sets a key value.
This new setting is in addition to the old cookie prefix.
