I've found that there are a few sites which I an unable to log in to. I get to the login page, fill in username and password, but I just get returned to the blank login page again.

Using Windows/IE, I am able to login, but not with Slack/Firefox 1.5 or Konqueror. I have java plugin enabled according to "about : plugins", and the verification at java.com works, so I doubt it's a java problem. I also have the Shockwave plugin installed.

I've tried using the User Agent Switcher extension to be seen as IE6, but that didn't help. It doesn't seem to be a FF problem, because it happens with Konq also. Cookies for the relevant sites are enabled. Any Ideas?

Using Windows and other browsers can you get in? Like FF, Opera, Netscape, Mozilla... My guess is you need Active X, and only IE can do that. If you don't mind us asking though so we can try/verify and possibly help, which sites give you said issues? I have never had those, even with my banks.

I installed FF 1.5 on a Windows machine and I was able to log in. So far it only seems to be a problem with login pages.

I can't really give examples because I would have to give my passwords to duplicate the problem, but there is a demo site I can give: http://www.bluehost.com/tell_me_more.html

Click on the "demo login" link near top right. When I'm using the Windows machine it says "logging in..." for a few seconds, then opens the demo. On Slack a window opens on top of the "logging in.." screen asking for username and password. Obviously I don't know what to fill in as it's supposed to be an automatic entry. No matter what I type in the fields, the login screen reappears empty after each attempt (no error). This is what basically happens with the other sites too.

I am using FF 1.5 on SuSE 10(OpenSuSE) right now and I am auto logged into this page: http://209.63.57.141:2082/frontend/bluehost/index.html.
The same happens on windows, and slick. I also tested with Opera and Konquerer on my SuSE box, all work.

I tried on my SLICK laptop with Opera, FF, and Konquerer, and again, all work.

I would have to say you may need someone with slackware to take a look, but may like SuSE 10 Slack has a firewall. Disable it if I am right, and it should work just fine. Otherwise it is a slackware problem, not a linux or FF or Konquerer problem. Did you use the FF that came with slack or install another? Maybe it is the distro installed FF that isn't working, and installing another version from source or rpm would work.

I second RedShirt. I also can login into the Demo login page without a problem and it must be a firewall problem related, maybe you have some special ports closed used to login.