Thinking of switching to Ubuntu...couple of questions?

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http://askubuntu.com – I am planning on installing Ubuntu later today, but I had a few quick questions first... should I back up my files before installing it, or will I still be able to access them? Will my bookmarks on Firefox still be accessible? Does anyone have any tips/is there anything I should know before installing it/any problems I might run into using it? Thanks :) (HowTos)