Being heavily involved in the production of Beltane Fire Festival we regularly go through quite alot of fire retardant spray every year
What I can say about it is this:
Its borax-based, and is quite a nasty chemical when wet - the instructions all recommend use of gloves, well-ventilated areas, etc
It only works for those materials that are slightly absorbent - it won't work as a surface coating on say goretex or waterproofs.
It works by drying out, leaving borax crystals trapped in the fibres - these stop small heat spuirces causinng ignition (i.e cigarettes, sparks etc) but will not stop material burning if held to a lighted flame. It also doesn't in any way stop the heat transferring to the cloth, it just reduces its ignition risk, so hot sparks that stick to fabric will still smoulder and burn holes through the material.
If the material is wet, the fire retardant leeches out - if it is very wet, this leeches onto the person wearing the fabric, and the weak boric acid causes skin irritation and minor burns
I've experienced this wearing a fireproofed cloak in the rain and it itches like mad
All in all, I would probably just prefer to have small scorch marks on a fleece than boric acid on my skin - its messy to apply, has to dry for 24 hours, stops working if it rains, and won't really stop small black scorch marks appearing on fleeces anyway if you do get hit by a spark from a fire - it'd just make sure you don't go up in flames if your fleece is highly flammable.
(As an aside, at the festival we ban all man-made materials from performer's costumes, so wool/cotton only, and we /still/ fire-proof them, and they /still/ get marked and scorched
