There is a well known "bug" in firefox, whereby typing on the keyboard will start the "find as you type" (aka "lookahead find") feature and start searching the web page for whatever you typed. I believe firefox enables this option by default.
The firefox documenation (Keyboard Feature: Find As You Type) and others (How to Fix the Find-As-You-Type Bug in Firefox) instruct that in order to turn off the feature you need to set accessibility.typeaheadfind.autostart to false in about:config.
This made no difference for me with Firefox 1.5.0.9. Instead, setting accessibility.typeaheadfind to false turned off "find as you type" and still allowed me to search with / or '. Do this by typing about:config in the address bar, hitting return, then double-clicking on the accessibility.typeaheadfind until it's value is set to false.
As pointed out at How to Fix the Find-As-You-Type Bug in Firefox, this setting may be being overwritten by settings in your user.js file that's located in your firefox profile directory (.mozilla/firefox/[profile] in linux). Move this file out of the directory to prevent it having an effect. The user.js file may have been created automatically by an extension, so if you still have problems it could be worth disabling all extensions and re-testing (do this from the extension manager by right-clicking the extension and selecting the disable option).
