Firefox 4 small window pops up everything else is grayed out

Question

I noticed that in Firefox 4, when I click to empty trash in web mail, the background gets grayed out like so:
[http://www.c627627.com/IMG/Other/Firefox4GrayBackground.jpg Screen Shot here]
I would like to disable that because everything time it happens - it makes me think that something's wrong...
How do I disable this behavior and have the window pop up but not gray out the background, which is how it used to work in Firefox 3.
'''EDIT: Thank you Gingerbread Man for your help. After thinking about it, I think the best approach is not to disable it but to simply prevent it from changing the background to dark gray. Adding this to \chrome\userChrome.css file accomplishes that:'''
tabmodalprompt {
background: transparent !important }

Answer 1

EDIT: Resolved here:
''You can go to about:config and switch a preference to false, but then you'll lose the ability to switch tabs while the prompt is open.
KWierso
prompts.tab_modal.enabled
Thank you. Yes, you may not click on other tabs while the prompt is open [now I understand why they did this].