I'm having the same problem in LM12 with cinnamon in both FF10 and Chromium 16 browsers.
My other set up is Crunchbang 10 bpo with IceWeasel 10 and Chromium 9 and both updated fine and work as they should. It would be interesting to know if either of the LMDEs work. Very tempting to reinstall just to see. I'm thinking that it is an LM12/Ubuntu 11.10 issue, but I'm not sure.
Sorry I'm not much help, but at least you know you are not alone.
Mint 14 cinnamon 64 bit
#! Waldorf 20130119 64bit