A smart keyword is stored as a bookmark, so you just need to delete it.

Go to the Library (Bookmarks -> Organize Bookmarks or Ctrl+Shift+B)

In the Search Bookmarks textbox, enter in the title of the keyword search bookmark. You can see the title in the location bar if you use the keyword.

Then, just delete it with the Delete key or right click and choosing Delete.

Thanks for the reply.

However there are certain keywords that do not appear when I type them into the location bar. But when I hit enter, they still take me to a search page. They don't appear to be in my bookmarks anywhere either.

Here's a screenshot of the only way I know of being able to see them all, which is with the Vimperator add-on.

I have no idea how 'bugmenot1' and 'urban1' got there, and I can't find them in the Library.

Try :dialog searchengines.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window to disable all extensions. You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")