Your line is spinning on your spool .
To correct this .
Take 15 or 20 wraps of Mono line , USe a blood knot to Connect the Mono line to your Fireline , and sinch the tag ends , and clip it and respool as you normally would . This will stop the slipping .
And here is how you tie a Blood knot .
1) Lay the lines parallel and facing in opposite directions and cross the end of one over the other. Wrap this line three or four times around the other and place the end of this line in the crotch or the V formed by the two lines.
2) Do the same with the other line, being sure to pass the end through the loop in the opposite direction to the first line.
3) Slowly pull on the standing ends of the lines to draw the knot together. When the knot is formed, lubricate with saliva and pull the knot up tight.
4) Trim the tag ends off so that they don’t catch in the rod guides.
And a little picture for you .