Can't open links in Firefox by clicking URL in Thunderbird

Question

This has started happening since the last Firefox upgrade (to 14.0.1). It affects two machines - one running Windows 7, the other XP. The symptoms are identical.
Firefox is the default browser on both machines. I'm using only a default profile on each machine. I've checked that there are no stray "no-remote" switches in the command line or environment variables and I've tried deleting "parent.lock". The fact that it has happened to two separate installations, as a result of the upgrade, and the fact that all other functions of Firefox work as expected, makes me doubt that it's a profile corruption. I have also tried opening Firefox in Safe Mode but the behaviour is the same.
Here's what happened/happens:
Previously, if I had opened Firefox in the normal way from the start menu and then clicked on a link in an email in Thunderbird, the link would open in a new tab in the running copy of Firefox. I could do this with multiple links, each opening a new tab.
Now, if I do this I get the "Firefox is already running" error box.
If I close Firefox the click on a link in an email in Thunderbird, it will start Firefox as expected, and open the link. But if I then click on a second link, I get the error message.
If I click on URLs in other applications (for instance Pidgin instant messenger), the links open in new tabs in the running Firefox in the usual way, so the problem seems to be specific to links opened in Thunderbird.
As an experiment, I rolled my FF installation back to 13.0.1 on the XP machines. Everything then worked as before, and as expected, so this is clearly to do with the upgrade.

Answer 1

Hi tlaclair, when I click that link, here is what I see (observed using the Fiddler2 program, which intercepts web traffic between your browser and websites):
(1) The server at link.p0.com sets 1 cookie and redirects to http://t.mookie1.com/t/v1/event?migClientId=181&migAction=email-click&migSource=mig&migDest=http%3a%2f%2forder.dominos.com
(2) Firefox follows the redirect and the server at t.mookie1.com sets 3 cookies and redirects to http://order.dominos.com
(3) Firefox follows the redirect and the server at order.dominos.com redirects to a secure session ([https://order.dominos.com/])
These kinds of redirects are completely standard; if you set Firefox to warn you about redirects, then a bar usually appear below the address where you can click an Allow button to continue. You can check whether you have this feature turned on here:
orange Firefox button ''or'' classic Tools menu > Options > Advanced > General
If Firefox isn't blocking the redirect, it's possible that one of your add-ons is doing it. A standard diagnostic for interference by add-ons is to try Firefox's Safe Mode.
First, I recommend backing up your Firefox settings in case something goes wrong. See [[Backing up your information]]. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)
Next, restart Firefox in Firefox's Safe Mode ([[Safe Mode]]) using
Help > Restart with Add-ons Disabled
In the Safe Mode dialog, do not check any boxes, just click "Continue in Safe Mode."
If the link works correctly, this points to one of your add-ons or custom settings as the problem.
Any change?