Spotlight makes a great deal when it comes to searching inside all the documents on your hard disk. Unfortunately it is not able to index Firefox' bookmarks by default (It does index Safari bookmarks though). Especially if you are used to bookmarking lots of sites this would be very helpful.
One possibility is to convert your Firefox bookmarks to .webbookmark files which are being recognized by Spotlight. At HubLog I found this nice little Perl script which will do the job for you. Unfortunately it has been written for Firefox 2.x and uses the bookmarks.html file. From Firefox 3.0 upwards this file is not created anymore by default (bookmarks are stored in a SQlite database). But you can still enable additional automatic export to bookmarks.html as described here (LyteByte). After enabling this, you should close Firefox and fire it up again. After you can run your Perl script and voilà: Spotlight finds your bookmarks!

converted .webbookmark files are stored in YourHomeDirectory/Library/Caches/Metadata/Firefox and opened with Safari by default (since Firefox is unable to read them).

I also added a cron job which executes the script once a day.

Thanks to all above mentioned Blog authors for their contributions!