https://addons.mozilla.org/en-US/firefox/addon/popupoff/
Popupoff simply removes those pop ups and since you can’t see the dialog to accept the cookies it is like not having given consent to cookies. (It sometimes makes the site not work properly, but you can set global and site specific to mitigate the issues.)
I even asked the developer here to make sure.
I also managed to install it on my android, to do this follow this guide https://blog.mozilla.org/addons/2020/09/29/expanded-extension-support-in-firefox-for-android-nightly/
I think the timer on each device starts from when the person who has the devices sees the message.
So if you send the message and the timer is 5 minutes, the message on your account (on all of your devices) will be deleted in 5 minutes from now, while the recipient will first see the message (maybe in an hour) and then after 5 minutes it will be deleted from their devices too.