You probably use your own spell save DC, but RAW is unclear
The description of the Ring of Shooting Stars (DMG, p. 192) does indeed not give a spell save DC for faerie fire. It also gives access to dancing lights and light, the latter of which may also require a spell save DC ("if you target an object held or worn by a hostile creature"), which the item's ...
Rules as written (and excluding house rules or specific rules for the effect that called for a save) there is no added effect from rolling a 20 on a saving throw (nor ability check).
The general description of saving throws in the Player's Handbook (p. 179) make no mention of rolling a 20 (nor a 1). However, the DMG section on ability checks and saving ...
No, you do not negate extra damage.
When making a saving throw, there is no such thing as a critical success nor critical failure. When you roll a 1 or a 20, simply add any relevant modifiers to that saving throw and use that total. The only exception to this is with a death saving throw (see basic rules, p. 76):
Whenever you start your turn with 0 hit ...
The Huge or Larger creature can move through the sphere
The spell states:
On a successful save, a creature is ejected from that space to the nearest unoccupied space of the creature’s choice outside the sphere. A Huge or larger creature succeeds on the saving throw automatically.
So the creature can choose to go back to the square it was just in, or to ...
You save against Ball Lightning but not against spark shower.
DMG, p. 214:
Magic items produce spells or spell-like effects. For a saving throw
against a spell or spell-like effect from a magic item, the DC is
10+the level of the speell or effect + the ability modifier of the
minimum ability score needed to cast that level of spell. For example,
A victim typically doesn't make saving throws against the functions ball lightning and spark shower
The description of the ring of shooting stars (Dungeon Master's Guide 232–3) (50,000 gp; 0 lbs.) says that victims make a saving throw against the special function shooting stars but does not say that victims make saving throws against the special functions ...