Based on the rubber strap one, the straps are 9/32" rubber. A basic rubber has about a 1,500 PSI tensile strength, giving the straps about a 95 lbs break strength. Multiply that by 4 and you get 380 lbs of retaining strength on those straps.
A 5-lb fire extinguisher weighs about 9.8 lbs. Meaning you would need a sustained deceleration of 38G to break the straps. Instantaneous peak deceleration in a head-on crash can be a bit higher, more like 40G for a 35 mph hit into solid concrete, but that peak might not necessarily be long enough duration (~100ms or so) to allow the straps to get stretched to their limits.
https://www.researchgate.net/figure...mph-full-frontal-barrier-tests_fig1_233738149
Worth noting that although the 35 mph hit seems slow, it's actually fairly conservative for most vehicle crashes, since most crashes are not dead-on into solid concrete, but instead oblique against solid objects, against narrower objects (e.g., trees), against road barriers, or against other cars.