Yes, that's definitely suspicious.

Could you first check for an optional settings file named user.js file in your profile folder (personal settings folder). Firefox uses the settings in that file at startup to override your saved preferences from the previous session. Unless you created that file yourself, it may contains settings from an add-on or external software to make this modification. You can check and remove the file using the steps in this article: How to fix preferences that won't save.

Note: By default, Windows hides the .js file extension. To work with files as accurately as possible, I suggest showing all file extensions. This Microsoft support article has the steps: http://windows.microsoft.com/en-us/wi.../show-hide-file-name-extensions.

And then here's my suggested procedure for tracking down and cleaning up bad add-ons and other hijackers. I know it seems long, but it's really not that bad.

(For Windows Vista, 7, 8, 8.1, 10; XP is somewhat different)

(1) Open the Windows Control Panel, Uninstall a Program. After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help in smoking out undisclosed bundle items that snuck in with some software you agreed to install. Be suspicious of everything you do not recognize/remember, as malware often uses important or innocent sounding names to discourage you from removing it. Take out as much trash as possible here.

(2) Open Firefox's Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons
• in the Windows "Run" dialog, type or paste
  firefox.exe "about:addons"

In the left column, click Plugins. Set nonessential and unrecognized plugins to "Never Activate".

In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions. All extensions are optional; none come with Firefox.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

(3) You can search for remaining issues with the scanning/cleaning tools listed in our support article: Troubleshoot Firefox issues caused by malware. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.

There is software like Advanced SystemCare with its Surfing Protection feature that can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

There are other things that need attention.

The System Details List shows that you have multiple Flash plugins.

1. Shockwave Flash 18.0 r0
2. Shockwave Flash 20.0 r0

You can find the installation path of all plugins on the about:plugins page.

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

You can check the Flash player installation folder for multiple Flash player plugins and remove older version(s) of the plugin (NPSWF32) and possibly (re)install the latest Flash player.

• (32 bit Windows) C:\Windows\System32\Macromed\Flash\
• (64 bit Windows) C:\Windows\SysWOW64\Macromed\Flash\

@jscher2000, there is no user.js file in my profile folder. I also did what you proposed uninstalling a program. After the installation but nothing suspicious appeared when I sorted by installation date

Well, I found a workaround for this. I created a new firefox profile and now I'm using this. In this new profile, the issue is not existent. I couldn't find what causes the issue but at least I have avoided this annoying misbehavior. Thank you both for your help

Sounds good. In case you haven't already found this article about selectively transferring some data from your old profile to you new one, here's the link: Recover important data from an old profile.

This is rather a longshot. Do you set Firefox to clear History and Cookies when it closes down ?

I mention this because just sometimes on recent versions of Firefox that may cause problems that include getting the wrong startup page. It is probably not what is happening. However if you do set Firefox to clear History (A new profile will use the default setting and NOT clear History) and the problem reoccurs then maybe that is the issue.