I just called Feinstein's office and registered my opinion. According to the staffer, Feinstein supports the extension. Her office can be reached at (202) 224-3841.
I called Boxer's office as well. After dialing <3> to speak with a staffer, it rang several times and then hung up. (202) 224-3553.
I interned for my Congressman on Capitol Hill a few years ago. In my opinion, volume calling is the best way to actually get heard. Web forms, emails, faxes: much less so.
U.S. citizens,
Contact your U.S. Representative: http://www.house.gov/representatives/find
Contact your Senators: http://www.senate.gov
It's only your senators that matter at this point. Call both of them.
Don't let this stop you (or anyone) from calling, but it's going to take a lot to get Feinstein to switch her stance on this.
It's especially disappointing given how out of line her beliefs are with certain areas of California, but that's what often happens when a massively diverse state elects a single representative democratically.