Jbbrwcky: your solution unfortunately did not work. Both Sidebar and Leftcolumn are floated left and all margins removed, but Firefox still stacks them. I even removed all the padding to be sure it wasn’t a maths issue.
I fiddled with every width I could to no avail–I even made my main container twice as big as it needs to be, and STILL Firefox puts my left column beneath the sidebar.
I’m almost ready to re-do the whole thing with tables. Please, someone help!