A keyboard-triggered navigation or selection (e.g. using the keyboard arrow-keys to navigate/select items in the Search-bar auto-complete drop-down items in RHEV-M web-admin) does not work as expected in Firefox when the "Always use the cursor keys to navigate within pages" option is enabled.
This option resides in the Firefox 'Preferences' -> 'Advanced' -> 'General' -> 'Accessibility' section and is disabled by default. It is recommended to keep it that way when working with RHEV-M web-applications. Note that hitting the "F7" key in the keyboard while working in Firefox toggles the enable/disable state of this option as well.
More information on this Firefox option can be found at: http://support.mozilla.org/en-US/kb/accessibility-features-firefox-make-firefox-and-we#w_always-use-the-cursor-keys-to-navigate-within-webpages