Warning: this is not recommended at all, as it makes it easier to link your different browsing sessions. I’m actually mainly posting it here for future reference on how to disable it. If you really do know what you’re doing, read on.
It’s simply a boolean setting: in
permissions.memory_only to false if you want Tor Browser to remember your permissions from a browsing session to another. This can be used for instance when you’re tired of being asked for permissions to use HTML 5’s Canvas-of-Doom. Personally, I’d prefer an easy setting to just kill Canvas once and for all.