Using Firefox 57.0.1 (64-bit), I encountered a memory leak. This leak only occurs after triggering the switch for the Christmas seasonal event.
I went from 350MB usage for the browser, to 7.6GB usage. No other tabs were utilized before or during this period. I began to monitor it after it became slow to the point of unresponsive, finally crashing.
The event switch wears off in a few hours, I will report back if the memory leak persists.