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I've tried unplugging the router several times. I'm loth to reset the firmware as this will mean setting it all up again. Why should it only affect my Mb though and not all the other computers?

I could not honestly say, only that I've seen weird issues like this before and they were fixed by resetting the router or the modem or both. Even restarting the Mac has helped in some cases. It's all voodoo to me.

I'm living proof that it worked. I would try to explore every other viable option too before reinstalling OS X, but I personally couldn't find one.

If you have tried everything else, and if the issue can be traced even circumstantially to the Airport update, then I table my objections to a reinstall. According to the MacFixit article it seems some have corrected this issue by deleting a plist. Obviously this would be worth trying before resorting to a reinstall.
 
Re TDD's post of Solutions from MacFixIt (taken from an Apple Forum Thread), I've ALWAYS had this issue, even before the latest "Security" update.
Now, it's an ingrained routine: reception slows or drops, I click Airport on/off/on/off until a better signal strength & speed, if that fails then Restart &/or check permissions, ya-da, ya-da. This has been a recurring issue with the PB G4 I own now and the iB G4 I previously had. Somewhere in MacRumors, Mad Jew led me to either a sticky or a guide that held a long list of suggestions for speeding up one's Apple without undergoing major surgery. I'll look for it and post back.....
 
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