Read PSI's answer. Pressing '/' turns on the search but it won't do that when you are typing in a text area (like it doesn't work when I press / when typing this reply. Are you sure you're not accidentally not typing in the text area but clicked somewhere else on the page? Might help if you explain what you are doing and what blog you use.
What might be happening is this. Say you are typing in a text area on your blog, you want to make something bold so you move your mouse up to the bold button, click it and start typing again. You type '</something>' and the find pops up containing 'something>'. The text you thought you were typing in probably contains no new text - not even the '<' pressed before the /. This is a problem with the blog software, not Firefox. It should return the focus to the text area after pressing the button. I don't believe there is anything you can do to complete turn off this off though - it's more a case of either changing your browser (although Firefox is the best IMHO) or changing your typing habits.
To see the advanced options for Firefox, type "about
:config" into the address bar. The Type As You Find options start with "accesiblity.typeaheadfind". For further info, see here:
http://www.mozilla.org/support/firefox/tips#beh_taf