GMail authorization disappears intermittently when idle

Question

gmail

Hello. Recently, GMail has begun to “reset” authorization if the mailbox is open but not used for some time (10-20 minutes, I didn’t notice it for sure): instead of a list of letters, a login / password input window appears, and the draft of the letter being created disappears in a separate window.
Previously, this was not the case - the box could be open for several hours in a row and, even if it was not used, it would not close.
I didn't touch anything, I didn't change any settings.
Is there any way to disable these excessive security measures? Can I again make it so that authorization on this computer does not fall when idle?
(Windows 7, Firefox 29)
Thanks.