Do you have any data to detail this? Fiberglass is glass fibers + resin. Decent resin doesn't melt or become weak with normal heat, it just thermally stresses at some point (we're talking > 180C at which point a wood ladder would be on fire). Wood also has issues with heat - it expands, which can cause buckling and cracks. I'm trying to find fiberglass temperature-strength data to figure out why it's not used in fire ladders and am coming up short.
As far as I can tell after some brief googling when I saw this article earlier today: 1. Fiberglass ladders tend to use polyester resin. 2. polyesters tend to have a glass transition temperature (i.e. where the polymer starts to become soft and lose its shape) in the 75 to 125C range.
Wood combustion temperatures are typically about twice that. Also, incidental heat contact can char the surface of the wood and leave it mostly structurally intact.
The choice of resin can be accomodated, using polyamides as a matrix with suitable glass (some low-temperature types might not work), or with carbon fibers, should allow ~400°C/ 670K as a usage-limit. If it were to be coated with e.g. CVD SiO2 or something similar, it could have a pretty high reflectivity in the higher-temeperature thermal radiation range of light (where most of the radiation heat from a fire is in).
The mentioned charring is a very important property in buildings, as solid (hard-)wood pillars have astonishing endurance in a full-scale fire, due to them smoldering slowly from the outside in, compared to e.g. steel framing that quickly softens throughout. One additional factor is evaporative cooling/associated covering with mostly inert water vapor due to the water contained in the wood and the water released from the cellulose fibers upon thermal decompositon.
