Closing "Are you sure you want to navigate away from this page&qu

Hi,
Every once in a while I stumble onto one of these #%@^! sites that try to sucker you into installing spyware masquerading as anti-virus software using a bogus screenshot of your My Computer drives with virus alerts. There's no danger of them fooling me but the fact that they're able to throw up a modal pop-up in FF with the following message:

"Are you sure you want to navigate away from this page?
To prevent your PC from slowdown, press CANCEL.
Press OK to continue, or Cancel to stay on the current page."

and ignore my attempts to close the tab using CTRL+W or clicking on the tab's X freaks me out a bit.

The last thing I'm going do, of course, is do what they're trying to get me to do (i.e., click page to install God knows what), but I'm not able to close the tab without closing FF from the Task Mgr.

Are there any hard and fast rules for these situations? Is the always a FF message that is safe to click OK here? On the one hand, it sounds like OK will complete the action I started by clicking on the X, but I don't like the fact that the site was able to insert that 2nd line into the pop-up.

Is the only safe way to close these nuisances without closing FF?

Thanks.

unfortunately fact that the pages are often coded so that when you try to close the page it you get a popup

That sounds like you may have a slight infection or similar initially, nothing sinister as such from what you've said, but possibly a minor hijack attempt, albeit a minor one, my suggestion is to get some protection and run some scans to check and be safe, obviously you have your wits about you, thats a massive part of the battle, but even clicking a pop up to close it can "sometimes" be enough to do something.

Not knowing what protection you may have\run, I suggest the following tools incase you don't have them, needless to say update and run full scans initially, then use as part of regular maintenance say running 1 tool a week or similar for peace of mind:-
a-squared Free
Malwarebytes' Anti-Malware
SUPERAntiSpyware Free Edition

Once those are done to aid blocking such events in the future I suggest you get and use the following tools, you need to check them for updates and apply when needed, these tools will aid blocking known bad sites:-
Hosts (requires special procedure for vista and windows 7, it gives instructions on the page if needed)
SpywareBlaster (helps block flash based files)

I also suggest the following junk file cleaners for use in general maintenance, these will help remove cookies etc that can be used to track your usage and steer you to related type sites in searches etc, but as well as that they just remove junk etc and help make the system run smoother:-
Disk Cleaner Check the boxes, select Options and check the top box that says "Delete Locked Files…" then save and run, you only need to access the options once, but as you use tools for the 1st time with this installed more boxes will appear so check those when they come up to clear junk from those as well.
CCleaner Watch out for the list of check boxes during the install, remove the bottom one for installing the Yahoo toolbar, unless you want it that is.
ATF-Cleaner Especially useful for cleaning after an infection, the link to download it is in the small print text in the description, click on it's name for the download, hopefully this will make sense when you see it.

Why 3 cleaners you may ask, well simply I find each often find stuff the others don't, so you clear the most junk you can, none will remove files, work or anything you need (unless you have some very special requirements), they will simply empty the bin and remove temp and junk files which are built up during normal usage, should you use cookies to stay logged into sites these will be wiped, BUT, you can set the applications to save the cookies you need, I will explain further if you should want this.

Now I don't use Firefox for my main browser, so don't really know the best add in's to use with it, but I'm sure if you go to the Firefox site, you will find numerous additions for it to aid with blocking pop up's and the like, my best advice is if they have a most popular type listing then you should have a good idea of the most sort after extras, then with all this done you should find the issues you currently face far and few between, hope this helps.