I am on the latest version of both WordPress and Firefox as of this writing - namely v4.1 and v34.0.5 respectively and running on a TechPreview of Windows 10 (Build 9879 to be precise).
My main browser is Firefox, and whilst I also have Chrome and of course IE - I use them only on occasions of in some cases when I have to use them for one reason or another.
When trying to login to WordPress from Firefox, I just cannot seem to login and get the dashboard. I know the user name and password is correct, and I don’t get any error - but keep getting the login screen again. Logging in again with IE is not a problem. I can’t recall when this started - if it was when WordPress was updated or FireFox - but both have recently, and it is very annoying to say the least!
I haven’t had the time (yet) to try and figure out what is wrong.