Aren't you currently on a site where most of the content is more off-centre, and having a left sidebar would actually put it more in front of you?
I can sort of sympathise, I'm fairly 'OCD' (not actually, but as people say) about similar things, but it's never bothered me I don't think. Maybe try with it a bit smaller? I don't think mine's 'huge' - maybe a bit wider than single horizontally displayed tab would be (just as my choice for the length of title it allows me to read), a couple of inches on a 24"-diagonal monitor.
There's userChrome modifications that make Firefox's sidebar collapsible, expands on mouseover and covers the page instead of pushing the page aside. I use that with TreeStyleTabs. Mine when collapsed is only ~3 favicons in width.
I can sort of sympathise, I'm fairly 'OCD' (not actually, but as people say) about similar things, but it's never bothered me I don't think. Maybe try with it a bit smaller? I don't think mine's 'huge' - maybe a bit wider than single horizontally displayed tab would be (just as my choice for the length of title it allows me to read), a couple of inches on a 24"-diagonal monitor.