From the Quorum UFAQ:
The Corp has a Sandburg rezzed and 14c. The Runner has a Faust installed. If the Runner uses Sifr when they encounter Bulwark, what is Bulwark’s final strength?
Bulwark’s final strength once the runner has the ability to use paid abilities to break subroutines is 3. When Sifr is used, it is the most recently applied modifier, so even though Sandburg gives Bulwark +2 strength it still has a final strength of 0. Then when Bulwark’s ability resolves causing the Corp to go from 14c to 16c, Sandburg’s effect updates and becomes the most recently applied modifier, which applies after Sifr sets the strength from the base of 8 to 0; the final strength becomes 3.
Applying the same reasoning to Quicksand, the strength when breaking would be X (where X is the number of power counters on Quicksand); runner fires Sifr (assuming run happens on runner's turn), then corp triggers Quicksand, causing a re-evaluation of its ability.
Of course, if there's a parasite on it, it dies in between Sifr evaluating and Quicksand evaluating; the strength was momentarily 0. But strength at time of breaking, neglecting other factors, is X.
In the edge case of AOYCR runs, strength at time of breaking would be 0. Quicksand and Sifr occur simultaneously. Corp effects happen first on corp turns, so Sifr would get applied after Quicksand evaluation instead of before.