I recently ran into a problem. It affects mostly the Firefox of my Nexus 4, but also my Windows 8.1 tablet earlier today.
It just seems that the main MSE site does not accept the fact that I'm logged in. Regardless to re-logging in, nothing works. I can easily re-sign into gmail in private mode and then log in, and it works. But generally, logging in will just bring me back to the not logged in front page, or even worse -- it will identify me and tell me to refresh to page, to an unlogged in front page.
I tried clearing the cookies, and it worked for a whopping period of a day and a half.
The strange part is that the meta site worked just fine, but when I tried to post a question, whose content is essentially this, the phone decided that I'm not logged in on the meta site either. MathOverflow still worked as usually, but I don't think that if I tried to post stuff it would have agreed with me as well.
Did anyone else have this problem recently?
This seems to be a Google cookies issue, but I don't know, since it behaves really strangely. Maybe it's an SE issue instead.
EDIT: I tried clearing up the stackexchange cookies and the Google cookies, but it seems as if Firefox 36.0.1 insists on not writing them to the hard drive. Even when it logs in, several minutes later it will auto refresh and log me out.
EDIT 2: This problem continues. This happens on my phone (Nexus 4, Android 5) with Firefox 37 and still on my tablet (32bit Intel Atom, Windows 8.1) with Firefox 36.0.4
I found that the chat will absolutely refuse to let me log in as well from these devices. The http://chat.stackexchange.com/help page tells me that the cookies test fails. This is odd since this is essentially the only site admitting problems (although, truth be told, I don't browse a whole lot outside this website).