I can't recall anyone else reporting that AVG was doing this. Actually, it would be helpful to know exactly what AVG is doing. Your home page is stored in a file named prefs.js in your personal Firefox settings folder (your Firefox profile folder). Is it deleting that file? Replacing it from a "clean" copy it keeps? Has it marked the file read-only so that your changes are not saved between sessions?

Could you make a backup copy of your profile folder so you have it around for comparison after the scan runs? You might need to store it on a USB flash drive so it doesn't get scanned, too.

To access that folder, you can use

Help > Troubleshooting Information > "Show Folder" button

Navigate up one level to Ctrl+drag the folder to the copy destination.

I followed directions to verify the prefs script file location. When I try to view the contents I get an error condition as follows;

                          line: 1
                          character: 1
                          Error:   Invalid Character
                          Code:   800A03F6
                          Source:  microsoft JScript Compilation Error

I am assuming the file should contain script detailing user preferences?

Hi esmer, you got a little ahead of me.

The default action for a .js file is to execute it. Windows displays an "S" icon indicating that this is the default action. For your security, never, ever double-click such a file because you have no idea what it will do to your system. (I've written scripts to purge files and edit the Windows registry; some of them are serious programs.) Because the prefs.js file is designed to change settings within Firefox, thankfully it doesn't do any damage when you double-click it.

To view the contents of a script file, right-click it and choose Edit, which will open the file in Notepad. If the lines all run together, close Notepad, right-click, and try Open With and choose a more intelligent editor such as Wordpad.

Or, to compare two files, try WinMerge.

Thanks for the balance on raw enthusiasm!

Ok, I have tried the following;

I set up my preferences in firefox and verified them in prefs using notepad.

Specifically:

1: I chose my homepage and verified it had been changed in prefs...

      I then ran a full scan and verified the scan caused the homepage to be changed in prefs...

2: I again changed the homepage back to mine,

      I verified again it had been correctly changed in prefs, 
      I then did a firefox reset.

In both instances the change in prefs was to the build override homepage. it appears AVG's scan is causing a reset in Firefox?

Firefox's built-in Reset feature moves your existing settings folder to the desktop and I don't think AVG would be able to trigger that. But the effect may be similar, just without creating the backup of your old settings.

There must be some way to stop this in AVG or these forums would light up with many posts describing the same problem.

one additional item...

I went to verify the contents of my local prefs file again using windows explorer and noticed some unusual behavior. if I click on the file once to select it it de-selects itself in about 3 seconds, typically before I can right click to edit. I verified I have "double click to open an item" selected in explorer I've never seen that behavior in windows explorer before.

I've got the exact same problem. Even got the same error message as esmer when viewing the prefs script. Seems to happen every morning after AVG runs my daily scan. I hate having to redo everything each morning (been happening for a couple of days now). I'd hate to have to stop using either programs.

First, I need to say Thanks! to jscher2000 for all your interest in the problem.

so we're aware, I have AVG examining this from their end as well.

additionally,

I believe this started occurring with the upgrade to Firefox v19. is there a way to determine the date of the last upgrade?

I believe this started occurring with the upgrade to Firefox v19. is there a way to determine the date of the last upgrade?

Firefox 19 was just released on Tuesday, Feb. 19th. I'm not sure whether Firefox stores your exact update date/time somewhere.

I have this same problem. In addition, I've noticed that my profile has been moved from the Local Settings\Application Data\Mozilla\Firefox\Profiles\ directory to the Application Data\Mozilla\Firefox\Profiles one level up.

Hi KKStephens, there are two corresponding profile folders. The ones in Local Settings just contains cached files.

I also have this problem, I put a shortcut to prefs.js on my desktop after changing my home page, then printed prefs.js from the shortcut, I then did a whole computer scan with AVG, and before opening Firefox tried the shortcut, I got a message that the file had been changed or moved and could not be accessed, I then opened Firefox, closed it and rechecked the shortcut, this gave me the new prefs.js file. I presume AVG is either deleting or moving the file.

I had this problem with AVG Free. Every time I ran a whole system scan my Firefox home page and appearance would go to the default screen like I just signed up. This started with the most recent update. The summertime one.I uninstalled the AVG Toolbar App that showed up in my CCleaner program and that fixed it for me.

I had exactly the same problem (except I use Windows XP and AVG Internet Security). I got in touch with the AVG support guys and they told me how to exclude a file from any scan (see later). So I set up the scan to exclude the Firefox Profiles folder (in C:\Documents and Settings\username\Application Data\Mozilla\Firefox\Profiles in Windows XP) and this cured the problem.

To exclude a file from an AVG scan, do this: Open the AVG user interface and under Options, navigate to 'Advanced Settings'. Select Exceptions and then click Add - browse for the Profiles folder (location as above in Win XP) and select it. Then make sure you select the 'scan' option under the 'Use this exception for'.

That should solve the problem until AVG fix their software!

Cheers