Rashi on Exodus 15:14/רש"י על שמות ט״ו:י״ד

1ירגזון means THEY TREMBLE (not they will tremble).

2ישבי פלשת [REIGS SEIZED] THE INHABITANTS OF PALESTINE, because they slew the children of Ephraim who anticipated the end fixed for the period of slavery and left Egypt forcibly, as is explained in Chronicles (1 7:21) “[The sons of Ephraim…] whom the men of Gath slew” (Mekhilta).