You can look at the network.protocol-handler.external.mailto prefs on the about:config page.
If network.protocol-handler.external.mailto prefs are user set (bold) then right-click and Reset them to the default value.
You can also try to change the network.protocol-handler.warn-external.mailto pref to true.

You can also try: "Reset Download Actions"

• http://kb.mozillazine.org/File_types_and_download_actions

The first suggestions did the trick. THANKS!! I wonder how it got changed in the first place.

Worked for me also but did not know where the about:config was till I typed it in the address bar,the walla there it is, anyway thank you

I switched over to FF 4.01 today from Chrome as I thought I would give FF a try again. I don't have the mailto option in the applications link for settings.

It doesn't exist. How do I put it in there so I can click on emails and have it open up m outlook as chrome does? I will have to switch back to Chrome if this doesn't work as it would be too cumbersome.

thank you,

yep! Resetting network.protocol-handler.external.mailto to default did the trick! I've been wrestling with this on and off for days, trying to use Outlook as default for mailto in FF 5.0.

Thank you cor-el and wiiiindy!