1) would a bad FPR cause big backfires ?
Yes, if there is fuel being dumped into it that ignites along with what's sprayed in by the injectors. Blowing off the oil cap is a little excessive, don't you think? Your car needs to calm itself down =D.
2) is it possible to replace the FPR without removing the intake manifold ?
Absolutely.
3) if so, are there any special tools needed (the car is 40 minutes from home) ?
If I remember correctly, really the only thing you need is a 10mm wrench. Bring various sockets if you've got them. A flathead screwdriver. Really, the procedure is fairly simple.
Take a look at the PDF misha posted here:
http://www.matthewsvolvosite.com/forums/viewtopic.php?f=1&t=25331
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