Trouble opening links in Thunderbird

[Helder Terra]

I just found out about this, and I'm not even sure if it's a common Lucid 
bug, but here it goes:

Ever since I've installed Kubuntu Lucid both in my home and office 
computers, I've set Firefox as my default browser and Thunderbird as my mail 
client. And sure, Dragon Player is my video player too.

But, for some reason, every time I get a link in a mail message, or a video 
file attached, it doesn't run at all. Instead of opening the link on Firefox 
or the video file on Dragon, it opens a window asking me to choose a default 
program for the action.

It only happens with these two types of file. My office files open perfectly 
in OpenOffice from Thunderbird.

Now, Kubuntu have asked me to do this in the past, and opened a list of my 
menu applications so I could choose one. But in this one situation is 
another window. One that shows only my Home folder, so I can't locate the 
software I want to open the file.

Is there a way to make Thunderbird know what program should run each file? 
And it that windows of my Home folder, how can I locate Firefox or Dragon 
Player?

Thanks for the help.

Helder Terra