Perform the suggestions mentioned in the following articles:

• Clear Cookies & Cache
• Clear the Network Cache
• Make sure that you are using the "Remember History" setting as described in this article -> Settings for privacy, browsing history and do-not-track

-> Start Firefox in -> Diagnose Firefox issues using Troubleshoot Mode to check if your add-ons/extensions are causing the problems.

• Also see this article -> Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Check and tell if its working.

I just reinstalled my firefox so the problem is not with cookies or cashe. Though it might be some of my add-ons. Here's the list of what i use: https://addons.mozilla.org/en-US/firefox/collections/icegoblin/favorites/ I tried turning all of my add-ons off and it's not alerting me yet. But i'll have to wait some time as before that this alert poped up randomly. Nope :( that didn't help. Even with all my add-ons off firefox still alerts me!

Reset actions for File Types as described in this article -> http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions

Check and tell if its working.

There's non of those 2 lines in about:config. I deleted that file as instructed but the problem is still present ( I noticed that it usually appears when flash plugin or some script is involved.

Check if that's a Pop-up window. Perform the suggestions mentioned in the following articles:

• Pop-up blocker settings, exceptions and troubleshooting
• http://kb.mozillazine.org/Popups_not_blocked

Check and tell if its working.

It's not a pop-up window. It's a dialog window. If it appears it opens a blank tab after the last one with "bar:tabs" in the address line and gives me the alert. Then when i click "Ok" it loads the tab as it should be loading (not always as the last tab).

-> Create a New Profile to check if your current profile is causing the problems, as described in this article -> Basic Troubleshooting - Make a New Profile

• Now start Firefox using this newly created profile.

-> IF that New Profile works then you can recover your History, Bookmarks, Passwords,etc. from the Old Profile, as described in this article -> Recover important data from an old profile

Check and tell if its working.

Just yesterday I completely reinstalled my windows and even before that i had that alert problem too. But i had a bunch of other problems (that's the reason to reinstall windows). So do you think it is necessary to create a new profile? Wasn't it created when i installed firefox earlier today?

By following the instructions in my previous reply, you will be able to create a New Firefox Profile. It's to test if the problem is related to your current profile.

Creating new profile didn't help either ( Now I know for sure where to test it. It always alerts me when I use youtube links.

Perform the suggestions mentioned in the following articles:

• Troubleshooting Plugins
• http://kb.mozillazine.org/Issues_related_to_plugins

Check and tell if its working.

as you recommended before i tried to run firefox in safe mode with no plugins turned on. And even though all my plugins and add-ons were off it still alerted me. most recently on pages with scripts (f.e. google reader). But that could be just a coincidence as it doesn't always appear even after clicking the same link. i just noticed that it always appears with youtube.

Can you provide the specific links on which you see this error message ??

as an example, the link that comes to my e-mail when you reply: https://support.mozilla.org/questions/927736#answer-334782. Oh! and i think i got when it alerts and when not. if i click this link with mouse 1 button (left one) it goes ok, when with mouse 3 button (scroll button) (i use it a lot) it gives me an alert. And here's the link to youtube: http://www.youtube.com/?tab=y1&authuser=0. This one always alerts just as all the other youtube links.

I did not see any alerts while visiting those links..

Do you still get this error message when you try to open these links using other Web Browsers ???

see also: Register protocol

thank you

the only other browser i have is IE8 (wish i could delete it). And it seems there's no alerts when using it as far as i can tell.

Do a malware check with some malware scanning programs on the Windows computer.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.microsoft.com/security/scanner/en-us/default.aspx - Microsoft Safety Scanner
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy

You can also do a check for a rootkit infection with TDSSKiller.

• http://support.kaspersky.com/viruses/solutions?qid=208280684

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked