I have not tested what you describe.
However, adding three feet to a 1.8 m cable will not cause a Firewire cable-length problem. Maximum (total) supported Firewire cable length for Mac is 4.5 meters: http://support.apple.com/kb/HT2952
One more thing for your consideration, though: External (Firewire) iSight may not play well with multiple connected Firewire devices.
(More info: http://www.ralphjohns.co.uk/EZJim/EZJimpage6.html )
In my experience, Firewire compatibility problems are also possible with certain cables or adapters. Therefore, if you experience a problem in your first test, I suggest you disconnect all other Firewire devices during testing. You can then try at least one additional different connection alternative to determine whether your problem might be related to a bad or incompatible connector.
Mac Pro Quad Core (Early09) 2.93Ghz OSX 10.7.4 24" LED Cinema Display
iPad2 iOS 5.1 5th Gen AEBS
MBP 15" i7 (Early'11) OSX 10.7.4 External iSights
13" MBP (Mid'09) 2.26GHz OSX 10.7.4 iBook SE 366MHz OSX 10.3.9