Since Firefox 77, there is no config option for this.
There may be a more complex solution: <
## Before Firefox 77
`browser.fullscreen.animateUp` was removed at some point. Instead, you can edit the preference `toolkit.cosmeticAnimations.enabled`.
* * *
Found by searching for `anim` in `about:config`. (And also, in the good profile I clicked on the "Status" column to sort it so all the "modified" preferences were together, so I could see that this particular "anim" preference was one that I had changed).