With West Indian opening bowler Hayley Matthews continuing her remarkable form, taking 4-14 with her off-breaks, Brave set a modest target of 104.
Matthews' all-round powers were back on show in an unbeaten 35, and, along with Tammy Beaumont's 59 from 40 balls, the visitors sailed to victory with 26 deliveries to spare.
Fire are now in a strong position, due to their superior run-rate, to finish in top spot and qualify automatically for the final at Lord's on Sunday.
After Beaumont won the toss, Matthews dismissed in-form Danni Wyatt, Maia Bouchier, Naomi Dattani and Kalea Moore to leave the Brave tottering at 7-47.
Chloe Tryon singlehandedly rescued the innings with an aggressive 55 off 38 balls, adding 52 with Rhianna Southby, as the Brave were all out for 103.
Left-armer Jonassen, who captains WNCL side Queensland Fire, of the WNCL, and the WBBL's Brisbane Heat, finished with figures of 3-21.
Beaumont and Sophia Dunkley put on 34 in the reply until the latter was dismissed by Lauren Bell.
Beaumont reached her half-century off just 31 balls and added 72 with Matthews to take their side to a comprehensive win.
20°C