At least it's not just me with the 10MB update then! I've already had updates to Add-ons which weren't originally compatible with Firefox 3.5.
You can install PitchDark 3.5.0 which is compatible with Firefox 3.5 if you want to. It's called an 'Experimental Add-on' mainly because it hasn't yet been passed by Mozilla. You'll have to register to get it. I've used 'experimental' things twice and had no trouble, so it's worth a try perhaps?
Get it
here if you decide to go ahead.