Daft oversight having installed AddThis sharing module
Realising I should be more savvy in respect of sites like Facebook, and not just in the sense of actually joining (done, just), I tried out the AddThis module which promised automatically to add suitable buttons on to my content.
And so it did, but when I tested with Firefox 3.6 what happens when a visitor tries to share a node in Facebook before they are logged in, I got a surprise.
A new tab was opened and the whole browser window was resized vertically to suit the Facebook login page, resizing itself to that height every time I tried to stretch it. Only once I'd given up and closed the Facebook tab could I have control over the browser in the manner that I'm accustomed to.
After some searching of the module issues, I tried the same test in Safari. No problemo there. So I looked again at the Firefox preferences which I may or may not have tweaked recently.
It would appear that setting the "Open new windows in a new tab instead" option in Tab preferences is not a good idea in this context. Firefox creates a new tab for the Facebook login and the entire browser is subjected to the resizing.