It would be more comfortable to have a floating toolbar like current FB toolbar. It's easy to access anytime, doesn't matter how long did you scrolled the page.
Before you even discuss whether something should be a toolbar, or whatever - first you need to know what you want to put there. I'm very unclear what features/buttons would be always relevant and pertinent to where-ever you scrolled, that don't already exist by the item they serve.
You might also be interested in the keyboard shortcuts, which may provide much of what you are after. Any use?
Personally, my view is that floating toolbars only apply in a very limited number of scenarios.
This is probably down to personal preference, but I find floating fixed-position toolbars rather intrusive and reduces the usable real-estate in my browser.
I remove as many toolbars as I can from the browser (bookmarks, addons, etc...) and it would irk me if a website then forces one on me.
In the rare occasion that I need to access the top links while I'm deep in a page, I'd much rather have a shortcut key that scrolls me to the top of the page. For now, I'm doing Ctrl+L then ENTER which reloads the page (thus grabbing new notifications) and put me back where I started.
As pointed out by Bobby in the comments, Ctrl+Home scolls to the top of the page which is also useful since it doesn't require reloading the page and will still work if the URL navigates you to mid-page.
p.s. the initial GM script that predates the StackExchange™ MultiCollider SuperDropdown™ had
position:fixed, but there was some objection to that positioning. IMHO, you'd be hard pressed to convince the dev/design team to make the toolbar be displayed as such.