UPDATE 2007-12-21: The problem appears to be solved.
There’s this really irritating bug that I have noticed in Firefox. I have the latest edition for Windows, and I haven’t used it on my Mac enough to know if it happens there as well. One of the great features that Firefox initially advertised was the ability to block pop-up windows. There’s a simple check-box in the Options menu that enables this function. On my version of Firefox, that option will randomly get unchecked and I have no idea what is doing it. I have done a few searches on Google to see if anyone else is experiencing this problem with no luck. I haven’t really had the time to do any real diagnostic work, but it is a maddening bug.
Sit, Ubu, Sit! 
I have the same issue.. Googling does not give a lot of hits for this issue.. weird.
I am having the same problem – everytime I turn FF off, the button is unchecked when I restart. Any luck?
L
unfortunately not, and nothing online either. it’s not every time I quit the app, just randomly while using it.
I’m having the same issue. Found you thru a google search….. if you have any luck please reply…
check out a new post on this topic:
I think I can help. Mine was turning off as well every time I closed firefox. This was happening because mcafee was doing it so chances are you have mcafee too. Open your mcafee security. Next click on configure. Then click on Internet & Network. Then click on Web Browsing Protection Enabled. Then click on advance. Then check the pop-up blocker option and make sure you click apply not just o.k. Now when you open firefox check the popup blocker and it should stay on from now on.
I checked the McAfee solution and that seems only to apply when the McAfee Privacy service is installed/enabled. I don’t have the privacy service installed, but I’m still getting firefox’s popup blocker turned off.
I’ve uninstalled McAfee myself and replaced it with AVG Free, but I’m still having the same problem. It sounded like a promising possibility too.
This is coverd in a Mozillazine article. Mcafee puts a user.js file in your profile directory with the entry: user_pref(“dom.disable_open_during_load”, false); // Popupblocker control handled by McAfee Privacy Service
they even sign it 🙂
If this is the only entry in your user.js, you can delete or rename the file, else edit it and remove the nastiness.
wow, that is really quite despicable of them. thanks for following it up. I remember reading something about the user.js file in the mozilla link above, but I forgot about it when I uninstalled McAfee. That was my only entry in the file, so I deleted it and I’ll see what happens.
Wow! I’ve had this problem for ages and could never find anything googling for it. I found that user.js file and it had the offending entry so I deleted it. Popup blocking seems to work just fine so far. I’m glad you posted this.
I eventually just scrapped McAfee and switched to AVG Free. Haven’t looked back.
I don’t even have McAfee (have Norton anti-virus) and have the same problem. Anyone know of other applications that could be triggering this same issue???
This fix worked for me even though I do not have McAfee installed anymore. The file was still there and had only the offending code in it. I deleted the file and Firefox works fine again. Thanks.