I've been setting up my browser the same way for years–I want my screen real estate for productive purposes, not tied up in toolbars and icons and whatnot.
I've tried the SmartBookmarksBar addon a couple of times, but there's a longtime bug that makes it unusable for me. I have a few folders on my bookmarks bar that hold groups of websites that I'll “open all in tabs” on a daily basis. When SmartBookmarksBar is set to “autohide” the bookmarks bar (I *love* this functionality, I don't need to see the bookmarks bar until I move my mouse over to it), it breaks those folders–you can no longer select the bookmarks or options within those folders.
Tangential thought: I can't be the only one that ponders this, but I'm surprised I don't see this brought up as an issue anywhere. The trend for some time now has been toward a widescreen computing experience… so with all this extra screen width, why do software designers continue to take away our vertical space, which is already hampered by the widescreen dimensions? For example, I use the TreeStyleTabs addon to stack my tabs sidebar-style. But whether it's our web browsers with their propensity to add toolbars and status bars across the top, with extra padding for aesthetics, or Microsoft wanting to put a big fat ribbon at the top of all their apps now… well, pretty soon we'll be required to scroll just to see a dozen lines of text on the screen. Widescreen HD video is great… but I wish they'd stop restricting my vertical space in so many apps!