This is an idea I got from the Arch forums (so thanks to them), but I thought I'd post it here too in case anyone hasn't seen it.

It's actually very useful.

Install ubiquity, then choose add-ons and ubiquity preferences. It'll open a tab, and somewhere or other it'll let you input custom commands (I think it's changed since I did this, so I'm not sure how you initially set them up now - maybe someone can clarify).

Now you can add custom search commands to a Firefox keyboard shortcut. For instance, I'm using;

Code:
CmdUtils.makeSearchCommand({
  name: ["susewiki"],
  url: "http://en.opensuse.org/index.php/Special:Search?search={QUERY}&go=Go",
  icon: "http://en.opensuse.org/favicon.ico",
  description: "Search openSUSE wiki."
});

CmdUtils.makeSearchCommand({
  name: ["susebbs"],
  url: "http://www.google.co.uk/search?q={QUERY}+site%3Aforums.opensuse.org&btnG=Search&meta=",
  icon: "http://en.opensuse.org/favicon.ico",
  description: "Search openSUSE forums."
});

CmdUtils.makeSearchCommand({
  name: ["susebug"],
  url: "https://bugzilla.novell.com/buglist.cgi?bug_status=__open__&content={QUERY}",
  icon: "http://en.opensuse.org/favicon.ico",
  description: "Search openSUSE bugzilla."
});

CmdUtils.makeSearchCommand({
  name: ["man"],
  url: "http://www.google.co.uk/search?q={QUERY}+site%3Alinuxmanpages.com&btnG=Search&meta=",
  icon: "http://www.google.co.uk/favicon.ico",
  description: "Search Linux man pages."
});

CmdUtils.makeSearchCommand({
  name: ["linux"],
  url: "http://www.google.co.uk/linux?q={QUERY}",
  icon: "http://www.google.co.uk/favicon.ico",
  description: "Search Google Linux."
});
So if I hit my key combo to open Ubiquity, then type "susebbs help I fooed my bar", it'll search this here forum for the string "help I fooed my bar".

Obviously, if you want to use these yourself, you should change the .co.uk and the en. to something suitable.

And if anyone has suggestions for how to improve them, or for other useful Linux related ones, please post them.