investigate and possibly remove all the OS specific prefs

there are a few OS specific prefs that we default to linux, I think we could get rid of them so that people will get the same experience as firefox. all prefs that change browser behavior with no good reason should also go.

goal of the issue:

• find all the prefs and document them.
• remove all of them.
• middlemouse.contentLoadURL should be moved to misc group of BROWSER BEHAVIOR in the process.
• browser.tabs.loadBookmarksInTabs faq entry should be removed or at least updated to explain how to get that behavior back.

for clarity, the windows section shouldn't be touched, that's a different thing.