Captain Neil de Kock also makes it into the first XV along with Glen Jackson and Kris Chesney, while Fabio Ongaro starts on the bench.
Full-back Leonelli missed the last month with a calf injury and returns to face his former club.
Warriors coach Sean Lineen has named an unchanged side to the one that beat Ulster last weekend.
Glasgow go into the game as underdogs, but Lineen thought his side were capable of upsetting the odds.
"We have not lost for a while, certainly not at Firhill, but it's a huge challenge against Saracens, who are the form team in England at the moment," he told BBC Sport.
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