Yes, unless something is interfering, your change should be remembered permanently.

Could you try removing the extension named "File Convertor and Search Addon". You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then cast a critical eye over the list on the right side. Any extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious? If in doubt, disable or remove.

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

Any improvement?

If you do not remember installing that extension, then I also suggest checking:

(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 surface 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. If you're not sure, feel free to post program names or a screenshot of the list.

(2) You can search for remaining issues with the scanning/cleaning tools listed in the previously mentioned 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.

Hopefully that will flush anything on your system and in your Firefox that could be a problem.

jscher,

Your suggestion to remove "File Converter and Search Addon" seems to have done the trick.

What WAS that? Did I need it, I don't remember installing it.

Note that your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts. The user.js file will only be present if you or other software has created this file and normally won't be present.

You can check its content with a text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete the user.js file if you didn't create this file yourself.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

WestofYouLB said

What WAS that? Did I need it, I don't remember installing it.

I doubt you needed or will need it. Please see my earlier reply starting with "If you do not remember installing that extension, then I also suggest..."