One of the first things I do when I download a new Firefox, is to update the Google search plugin to return 100 items on a screen. (Though Google often offers relevant results in the top 10, I find I'm as often searching to find "what's out there" as to reach "the best result.") Since I didn't find instructions the first time I googled this hack, here it is:

Find the "searchplugins" folder -- on Debian, it's in ~/.mozilla/searchplugins, while on Windows, it's probably C:\Program Files\Mozilla Firefox\searchplugins, and open the google.xml file in your favorite text editor. The guts of the search query is in the lines:

<Url type="text/html" method="GET" template="http://www.google.com/search">
  <Param name="q" value="{searchTerms}"/>
  <Param name="ie" value="utf-8"/>
  <Param name="oe" value="utf-8"/>

  <Param name="num" value="100"/>

That's a little "search suggestions" script to suggest terms you might want to use to complete your query (example) -- but you might remove it if you don't want all your half-typed inquiries sent to Google. On the browsing privacy front, you'll also want to examine the anti-phishing feature. The basic version checks sites locally (updating a local list with periodic downloads), but the extended version sends Google the URL of every website visit, to check against Google's list of badware.

You'll probably also want to disable the automatic installation of new search plugins, lest the change get overwritten next time there's an update. (Options, Advanced, Update)

I particularly appreciate the new "resume browsing" feature, given the number of times 20-tab windows have crashed on me...:

Resuming your browsing session: The Session Restore feature restores windows, tabs, text typed in forms, and in-progress downloads from the last user session. It will be activated automatically when installing an application update or extension, and users will be asked if they want to resume their previous session after a system crash.