The spell-check language setting on Firefox keeps changing to en_CA rather than the default of en_US. I change it back but it invariably flips... maybe in a few hours, maybe a few days. (It's hard to tell exactly) Nothing else on my system is Canadian, I don't know where it gets the idea.
At some point I suspected the profile: the wrong setting was in the profile but the profile was getting corrupted somehow so my changes weren't being saved. I started a new session, changed the settings, and closed the browser down to let it save the profile correctly. On restarting I was happy to see that it was correctly using US English, but then saw it flip back to Canadian.
My system is Ubuntu 12.08 with Firefox 18.