First of all, Fireline stays on your needle more easily. After you thread it through the eye and fold it over, it just isn’t likely to go anywhere because it will hold the crease of the fold. Nymo, on the other hand, is much more likely to accidentally slide off. You may have noticed, if you saw the How to Make Wrap Bracelets video, that I strung double the needed amount of Nymo and folded it over on my needle. This was to prevent it from slipping off as it is suseptible to doing. This tendency to slip off is especially frustrating because Nymo also tends to fray at the ends. This makes it very difficult to rethread on the needle. When working with Nymo, you may find yourself repeatedly trimming the frayed end of your thread in order to rethread your needle. Fireline, however, will not slide off your needle and the end will not fray.
Fireline is stiffer than Nymo. A stitch made with Nymo will have more flexibility. Because Nymo is nylon covered with wax, it will have some stretch to it as well, which Fireline will not. If you pull very hard while stitching, Fireline is probably the better choice for you.
Fireline comes in “Crystal,” which is clear and is not meant to be seen in your finished piece. Nymo has the advantage if you want a unique color of thread for your piece.
In a nutshell, I still recommend Fireline 9 out of 10 times. Nymo works great for Wrap Bracelets because you can obviously see the thread and the color needs to compliment the bracelet. However, the stitch I’m working on now for our next instructional video, for example, needs to be pulled tightly, is intricate and should not show thread. Plus, I can’t deal with the frustration of learning a new stitch AND fighting with thread that keeps falling off my needle!!!