Well, thats not exactly readable, let me try to reformat the Wireshark excerpt:

• 1 0.000 remote-collab > http [SYN] Seq=0 Win=65535 Len=0 MSS=1260
• 2 0.023 http > remote-collab [SYN, ACK] Seq=0 Ack=1 Win=5840 Len=0 MSS=1380
• 3 0.023 remote-collab > http [ACK] Seq=1 Ack=1 Win=65535 Len=0
• 4 0.033 remote-collab > http [FIN, ACK] Seq=1 Ack=1 Win=65535 Len=0
• 5 0.056 http > remote-collab [FIN, ACK] Seq=1 Ack=2 Win=5840 Len=0
• 6 0.056 remote-collab > http [ACK] Seq=2 Ack=2 Win=65535 Len=0

- Ole (again)

Try clearing the firewall permissions for Firefox, and then allowing the new version of Firefox thru the firewall.

The top 12 ways to get fooled by Firewalls:
http://forums.mozillazine.org/viewtopic.php?p=2295421#p2295421

Hm, interesting. This is what happens:

• When I delete firewall permissions for Chrome, and the restart Chrome, the firewall asks me if I want to permit Chrome to access the net.
• When I do the same for FireFox, I am not asked. Something is evidently blocking FF before it reaches my firewall. I've looked at "The top 12 ways", and am now trying if there's another firewall running or something.

See if anything here helps you. The 17 most popular firewall programs are covered. And the Enum Process program is explained a bit, along with a download link to that program. It is a good program written by a Firefox support helper over at MozillaZine to help Firefox users determine what they have running as far as firewalls and other security programs go.
http://support.mozilla.com/en-US/kb/Firewalls

The biggest problem with Firefox and firewalls is that when Firefox is updated it almost always comes with a new firefox.exe file, which is what launches Firefox and what the firewall looks for as being different than expected. Most other programs don't change their "executable" file almost ever, so the firewall is never challenged. Plus some firewall programs aren't updated in a timely manner and don't recognize the new executable automatically and need the users intervention (or jacking around, as I see it).

That's about the limit of my experience with software firewalls. I use an IP tables hardware firewall on my LAN, and only ever used a software firewall on a laptop I had a few years ago - and only because it was used off of my LAN. My current netbook uses Linux and I don't see the need for a software firewall, yet.

EnumProcess sees only my regular firewall, and since it does not help to disable or completely exit the firewall, there must be something else, blocking FF but not the other browsers. I even tried manually adding firefox.exe to program control, but still nothing.

Interestingly enough, FF loads pages fine in windows failsafemode. So some program must be blocking it. Is there any tool in windows XP that might help me determine which one it is? I guess in Linux, I'd be using lsof, but I don't know if theres a pendant in windows....

the-edmeister, you have been a good help, but unfortunately I can't mark the problem solved yet.

Exactly which firewall are you using?

F-Secure

Well this support page is like 3 years old, the interface you're seeing is probably different now in 2011.
https://support.mozilla.com/en-US/kb/Configuring%20F-Secure%20Internet%20Security

See if this helps you at all with the current version of F-secure.
http://www.f-secure.com/en/web/home_global/support/article/kba/2261

I appreciate the help, but really I have done all those things. It looks to me like there is something other that F-secure, blocking FF. Remember I did disable the thing with no improvement.

I am starting to suspect malware, here. But I've run a scan without finding anything, and since the condition is 6 months old, I suppose the virus databases should have caught up with whatever it might be.

Rest assured, I will be back when/if I find a solution. Meanwhile Google Chrome is a nice browser too :-)