I've had this discussion with my dad a few times – basically whenever the situation comes up on TV or in a movie. One can light a fire using a magnifying glass. I've seen it done (actually on an overcast day), and it's really neat. Magnifying glasses have strongly convex lenses. This is why they work.
Spectacles, however, do not work this way, at least not effectively. For nearsighted people (like myself) corrective lenses are slightly concave, so they don't focus the light like a magnifier. Farsighted lenses, reading glasses, and bifocals (like mine) are convex, but the curve is extremely slight. Optometrists recommend the weakest magnification necessary for reading glasses and bifocals. I have read anecdotes of people starting fires with spectacles, but the lenses would have to be large, the sun high and bright, and it would take a long time to produce even smoke.
Basically, no one's prescription is "strong" enough to start a fire. However, I postulate that if such a person does exist, perhaps a better sighted person should be doing the fire-starting…
Below is my original draft of panel 3. (The joke didn't "hit" for me, so I thought on it a while and drew a new punchline.)
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