Weird spotlight issue

This happens whenever I erase the free space on my hard drive (probably for the same reason -- that process creates a big file that eats up all the free space, then overwrites it, creating a low space condition in the process). Typing this command at the terminal:

sudo mdutil -i on /Volumes/Hard_Disk_Name_Goes_Here

will rebuild your index. Substitute your own disk's volume name in the obvious place.

You can rebuild the total spotlight database by adding your entire drive to the privacy pane in system preferences. Although that might be needed, you could try just adding and then deleting the folder than contains Firefox to see if that helps. This avoids missing all spotlight results while the entire process completes. A rebuild could take anything from 3 to 12 hours, depending on several factors.

Do be sure it's missing by showing all results - if it's not a top hit, rebuilding won't help. In that case try moving it into /Applications or perhaps looking into alternate app launchers (like LaunchBar) that allow you more control over where apps can be stores and more control over regenerating indexes.