Well that wasn't hard to fix Rated 4 out of 5 stars
OK so I just had a eureka moment:
Deleting all the cookies in thunderbird cleared the sign in page for me and i was able to sign into the other account.
great extension, thanks.
5 stars if the behaviour of clicking on links is made more predictable.
sometimes it opens a new tab in thunderbird (which is blank), other times it opens a tab in my browser (firefox), sometimes it loads in the current g+ tab.
Lubuntu 12.04, 64-bit, BTW.