I can't login in Stack Overflow using my Stack Exchange account from Firefox 21. This happen both in Windows 7 and GNU/Linux (Ubuntu 12.04 LTS 32-bit). I don't have any problem to login in Stack Exchange or other websites as:
Apparently the issue is only with http://stackoverflow.com.
When I try to login I'm sent to the homepage. I can't see any error message unfortunately. I've seen other users are suffering a similar issue, but related with the use of AdBlock Plus. In Windows I'm not using AdBlock Plus. I will try later today to disable it on Linux and see if that helps.
Any idea of what can cause this malfunction? In Google Chrome everything works smoothly.