I know how to map single key strokes to vimium commands, but I would like to be able to have 'dd' for closing a tab instead of 'd'. It seams like this should be supported given that 'gg' is mapped to 'scrollToTop'.
I figured it out after reading the source. It wasn't working since the default for 'd' (scrollDown) was still there. The logic takes the first available match so even though 'dd' was in the mapping, it could never get there. The solution was to 'unmap d' before 'map dd removeTab'.