Fireline is a pre-waxed, braided cord consisting of gel-spun polyethylene which is known as the strongest fibre, per diameter, ever created. It has an unbelievably high tensile strength and has been recommended in numerous how-to articles on beadworking. The 4 & 6 lb. tests will fit a #12 English beading needle, whereas this size may need a size 10 needle.
0.17mm (0.007") thick, with a break strength of 8lb, these reels have 45.7 metres (50 yards) length.
Whilst some prefer Fireline as their thread of choice, others prefer Nymo, and some people use both! It's always good to try them all out. It's definitely a personal choice.
Do make sure you use a really sharp pair of scissors, as this Fireline is much more difficult to cut than other threads.
When threading your needle, try squishing the end of the line flat in a chain nose plier, and this will make it easier to thread!