[EDIT]
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Thanks for your VERY reactive answer <3
Meanwhile, I found a trick that is even better in my case ; I set it up on 2 minutes every 15 minutes, and does not turn off the page (so if I stay with my work group and do not change conversation, I can still work.
Omg, I REALLY DO love your add-on !! Wish I could give more stars !!
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Does the job but a bit too perfectly as I set it up. I needed to stop Facebook. Well, it's done, and I'm glad with that.
But I need facebook/messages (Messenger) to keep working with some student groups ; is there a way to add some exceptions?
Thanks!
(I'd contribute if my bank account would allow me to.)