I'm ditching Chrome, finally worn out by its persistent memory bloat, and trying to move to Firefox (19.0). I have not imported/transferred anything from my Chrome settings.
On trying to log in (using my Stack Exchange OpenId), I got the message
Adblock interferes with login, please disable it
The only thing I could see remotely connected with ad blocking was Ghostery, so I disabled and tried again. Same result, but at the same time I have no problem logging on through Chrome.
In accordance with recommendations I saw elsewhere on this site, I cleared cache/history and cookies, but the problem persists.
I have also run the adblock check, and the reply was
What setting do I need to tweak in Firefox?
How do I do that selectively for Stack Exchange? (As I'm not prepared to open up for other things, in case you're wondering.)
Conclusion: I have no idea what the problem was, but by disabling and re-enabling my Convergence extension, things are now working. I'm sorry -- please feel free to delete as too localised.
I can confirm that it appears to have been something my Convergence extension was doing. Fortunately, I can leave it disabled.