Hi Quasi,
I use Maxthon as my main browser (which uses IE as its engine in the background). I dont' have a problem on my side with either the 2-month or 2-day duration period.
The only "strange" thing that happens is when I am on the 2-day candlecharts, if I click GO, I can briefly see the 2-month appearing in the "Duration period" box, but it reverts back to the 2-day duration.
So, I wouldn't say I have a problem with Maxthon (IE).
However, as you said, there seems to be a glitch with Firefox.
If I start with a 2-day duration, and then change it for a 2-month duration, I can't go back to 2-day. It doesn't switch. It seems to get stuck to the 2-day period. I can't go back to the 2-month duration period. But only with FF.
Hope this help you somehow.
Peter
PS: However, if you change your list, and come back to your previous one, you can go back to a 2-month duration... using FF.