URLs in TB email with latest Firefox version

I run (and have done for several years through various versions) the standard version of Firefox on two machines with the PortableApps version of Thunderbird on a USB stick which alternates between the two machines.

In both cases the Firefox installation is standard - just the default profile and it's the default browser on each machine.

Prior to last night's upgrade of Firefox, if I had Firefox open on the machine and clicked on a URL in an email in Thunderbird, it would open up a new tab in the existing copy of FF - which is as expected, and as configured.

Following the upgrade to FF 14.0.1 this no longer works. Now, if I click on a link I get an error message saying that Firefox is already running but not responding. In fact it's both running and responding.

I've tried starting it in safe mode, but the result is the same, so clearly it's not addon-related.

I've now rolled both installations back to FF 13.0.1 and everything works as it did before.

So between FF 13 and 14 something has clearly changed in the way it responds to links from TB. I'm wondering if this is related specifically to the PortableApps version and the way it handles those links.

When links are clicked within other applications (for example instant messages in Pidgin Portable), everything works as expected in both FF 13 and 14.

Any ideas? Anyone else experienced this?

Regards

Nick