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Possibly marginal/partially damaged CIA chip I/O pin logic (firebutton goes to CIA-A i/o pin).
Does it boot normally if you keep fire button pressed when you switch on power? You can also try swapping CIA chips. (CIA-A to CIA-B socket and vice versa) If problem goes away: damaged CIA chip.
I had A1200 with very strange parallel port (=CIA) problem. System crashed at boot (I think it was yellow screen) if printer was not connected! I debugged it a bit and found out system boots normally without printer if parallel port data pin (I think it was data bit 2) was pulled to +5v or ground. Can't remember (It was late 1990s) Pin not connected: boot crash. Also writing to parallel port data register crashed the system..
In other words: slightly damaged CIA chip can cause all kinds of weird side-effects.
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