GMail: Why am I suddenly unable to be logged in under two usernames at once?

I have several GMail accounts for different purposes. In the past, I’ve always been able to use two (or more) at once, opening each in a new tab (Firefox).

My GMail account on the Fluther domain just received an update, and now it won’t let me stay logged in under that account and any other Gmail account at the same time. It seems I have to choose just one login at a time. Very frustrating.

Does anyone know why this is happening and/or how to fix it?