Best answer: Try to disable the add-ons all and see it work on not. If work, then enable it one by one and check which one cause this problem. In my case, it is the skype extension 22.214.171.124.
I have discovered that this works in the cases of those instances of FireFox which exhibit the problem. But I can't tell all my customers to lobotomize their FireFox browsers in order to make the link I mail them work.
Does anyone have an elegant workaround?
(Note that cutting and pasting the URL into FireFox is elegant.)